When to hold him, or fold him?

When to hold him, or fold him?

My husband and I have been married for almost seven years, and together for almost 8 and a half.  I have two kids from before, we have one together and another due to arrive at end of August.  For the most part we have been happy, I thought.  We had our ups and downs of course, but we always came through and seemed to be stronger for it. 
His work this past year has taken him out of town.  He's away more than he's home.  He left the first time in January, on our anniversary no less.  I had just discovered a few weeks prior that I was pregnant.  We were both happy about it.  He didn't get home much and due to morning sickness all day and night I was in no condition to travel to see him.  He was about 3 1/2 hours away.  He came home only twice from Jan to beginning of March.  I was able to go see him once in mid March.  
While I was with him he seemed strange.  He had seemed strange even before then on our phone conversations.  I had asked him several times what was going on, he always said nothing.  When I went to see him he was different.  He kept his cell phone practically glued to his hip.  This was unusual.  He didn't seem to miss me as much as I thought he should.  He wasn't as affectionate as I expected.  I only stayed two days and two nights.  I left with the feeling something was wrong, but had no idea what it could be.  I thought maybe it was the pregnancy, and that he was a little weird about it.  Some guys get that way.
After I was home again, the phone conversations became shorter and shorter.  He'd suddenly 'have to go' and would 'call me back later'.  Then he texted me a few different nights that he didn't want to call and talk because he was 'so tired, would I mind if we just talked tomorrow?'  I tried to be a trooper thru all this, but my heart was aching.  I finally looked into his phone records online.
What I found was devastating.  I found hundreds of texts, and calls to and from a specific number. Naturally I blocked my number and called it.  It was a womans voicemail that I got.  My world shattered.  I waited a few days trying to come up with any good reason for all this, that didn't include cheating before I confronted my husband.   He played stupid about who she was.  Pretended to not know who I was talking about.  Even went so far as to deny knowing anyone by her name.  I was furious!  
It was the end of March when I confronted him.  The initial blow up passed.  He sent me a text telling me he had 'good news' that that particular job was over and he'd be home Monday.  this was a Thursday afternoon.  I later found out that his job had in fact ended that next day on Friday.  I tried calling him all day Saturday, and his phone was shut off.  A first.  Finally that night he answered my call, said he'd left his phone in his motel room on accident all day.  I blew up.  I accused him of what I was sure I knew.  I demanded he come home immediately, that I knew the job was over and he was lying to me.  He made axcuses why he couldn't come home til the next day.
When he got home we had it out.  He promised me that he was done with her.  When I asked him how he let it happen at all, he said he didn't know, that he had 'just never met anyone like her before'.  My heart broke even more.  With that statement he made me feel like she was special and I was 'just like everyone else'.  Again he swore to me it was over and that she knew it was because she knew he was going home.  He said she knew he was married.
3 or 4 days into April, I looked at his phone records again.  I saw that they had talked again since he was home.  He spent almost all of his time at home out in his shop, away from me and the kids.  I confronted him again.  He lied to me saying he had not talked to her.  I broke his phone in half.  We got him another phone and it got here a couple days later.  He was gone when it arrived.  I plugged it in and got it activated.  As soon as I did a text message from her popped up saying 'Missing you. Hope all is well.'  I didn't think it possible for my heart to break anymore!  When he got home I showed it to him asked him WHY?  He got all kicked puppy dog looking and said he didn't know.  
He was home for 3 weeks before he had to leave again.  That whole time he spent drinking, and hanging out in his shop.  He even locked the door a time or two, so we had to knock to come in.  When he left he promised me that he would 'be good'.  Two days later he texted her again.  When I asked him about it, he said he had told her to leave him alone, that he was through.  I asked him why he'd text her that when she had obviously not been bothering him the last three weeks he was home.  He got mad and said it was because he didn't want to put up with any more of this BS.  Meaning my confronting him and arguing.  I tried so hard to believe him, as she did not reply, and he did point that out.  At this time he was about 8 hours away, so seeing each other was hard.  I went at the end of April to visit him.  I found the text in his phone that he had sent her.  It was a voice text of a little kid saying 'I love ya Baby!' and he had added his own text to that saying 'I still do beautiful'  I wanted to die!  I didn't confront him with it as we were in a tiny motel room with our kids there.  I didn't want to have it out in front of them.
I drove home on Mothers Day.  I confronted him a couple days later over the phone.  He tried to lie about it.  I told him I had seen the actual text and that it was nothing like what he had told me.  He was silent.  I asked him if he loved her.  He said No.  I asked why he'd tell her that.  He said he didn't know.  From the time I found out about this affair, till this point, he had never given me any solid answers to anything.  Very upsetting for me.
Since then we've argued about it, I've cried about it, been mad about it.  He came home around Fathers Day for a job closer to home.  He was here almost two months.  Things were ok, not great.  He made it clear to me that this subject was off limits.  He would get mad at me when I'd bring it up.  Yell at me saying 'I thought we were going to get over this'  How can I when he won't answer anything I ask him with any kind of real answer?
He's gone again now.  6 1/2 hours away.  I am two weeks from my due date.  I can not travel to go see him.  He only has Sunday's off.  Not enough time for him to come home.  He's been gone about two weeks this time, and already his lies have started.  He 'went out' last Saturday night.  Ignored my calls and texts all night till I gave up at 11:30 PM and went to bed.  All day Sunday he wouldn't answer me calls or texts.  Finally Monday morning he sends me a text saying 'I love you and miss you, please don't be mad. I'm so sorry' .  As if!  We spent most of that day arguing.  Again, he wouldn't give me solid answers.
In the state I live in a woman can not even file for divorce while pregnant.  I checked in April.  There is no such thing as a legal seperation here either.  While I am pregnant, I am stuck in this relationship.  I try so hard to believe him when he says he'll stop.  That he won't do it again.  And this has as much to do with his excessive drinking since January as it does to the lies about where he is at night, and who he's with, if anyone.  The idea of divorcing makes me ill, especially with a newborn coming and three other kids to take care of.  But how can I ever trust him again?  He keeps on doing the things that are killing our mnarriage.  I fear that we are in a vicious cycle here and unless I leave, it'll never stop.  Is leaving the only option at this point?  
I appreciate anyone who has read this through to the end, I know it's a very long post.  I wanted to put as much information as possible in it so that anyone who may want to offer useful advice would be able to do so and actually have an informed opinion.  Please help me consider any other options that are out there besides leaving after this baby is born.  BTW it's a girl, my first!  Please help if you can.

Sincerely, Sharlo