I had sex with a girl yesterday (she was asian and about 5'1'' tall and really petite and also 18 like me and she wasn't a virgin) and when I took my dick out of my pants she was really scared and said that it's really big. The biggest she had ever seen (my penis is 8 inches long & 6 inches in girth). I said that I will be gentle. So after we did some foreplay (she was really wet and we also used a lot of lube) she said that she wants to try to take my dick inside. Btw. she couldn't get her fingers all around the penis so it was kinda too thick to her hand also. So I started to put my penis into her vagina, really slowly and gentle. It was really hard to put my penis into her vagina cause she was really tight in my opinion. I tried about 20 minutes to put it in and then I could only like put the head of my penis inside her vagina. Then she screamed really loudly and said that it's way too big and she can't handle it. I tried again and then I could put about a half of the dick inside when she screamed again, wanted me to stop and she started to cry. She said that it just hurts too much cause my dick is way too big.
So what should I do? Am I too big for her? And what should I do better next time when a girl is really tight? Cause I don't want to hurt and I don't want that any girl would scare my penis. I really really don't want to make a girl to cry during sex.

And why some girls have said to me during sex that they don't want to take my penis from behind cause it might hurt?