Give him a fair shot without punishing him for the faults of everyone who came before.
As much as I loathe clichés, there's a lot of truth in the one about kissing a bunch of frogs before you meet your prince. This isn't to say you're going to date a lot of losers or general trash; most of us date a few wonderful people who just aren't ultimately an ideal mate for us in the long run.
But whether you've dated a series of bargain basement f*ckboys or a lineup of guys worth their salt (or, like most of us, a combination of both), it's easy to let the defeat of "failed" relationships weigh heavy on your spirit and turn you into someone cynical.
I put "failed" in quotations for a reason, by the way. Here's the thing, and I'm sure you've heard this before, but I'd like to remind you: There's no such thing as a failed relationship.
Oh, the majority of romances don't work out, obviously. However, that doesn't at all mean that they "failed." Failure implies that you went into the situation with the expectation of a certain outcome, which is a mistake we have to stop making if we're going to be happy.
Those relationships that end in Hindenburg-sized disaster aren't "failures"; they're just lessons. Don't roll your eyes! I know it's a cliché, but it's true.
The magical thing about really god-awful relationships finally coming to an end is the transformation in you based on what you've learned about what you will and will not tolerate ever again. If you're like me, you've tried to resuscitate a long-dead romance for far longer than you're proud to admit, and the second you were free of it, you never again spent a second wasting time on anyone else who presented the same problems.
The trick is to leave your anger about that relationship at the door. That's easier said than done because after experiencing a bunch of partners presenting you with the same exact brand of bullsh*t, it's very easy to put up your guard when the next suitor comes along, because you've come to expect it. And, to be fair, it's not completely wrong to put up a bit of a defense at first; things are scary out there and getting to know and trust someone is not an easy task.
Take precautions if you're not comfortable at first, and preserve what you need to until you're ready to share all of yourself. However, once you let someone new into your life, it's important to give that person a fair shot without punishing him for the faults of everyone who came before him.
Honestly, a humongous majority of people out there aren't a match for you, but there are enough viable options for you to find real, enduring, comfortable love with someone perfect... if you give him a chance to prove it to you. It's up to you to give him the fair chance and purest, most open form of yourself that you both deserve.
Vulnerability is an exercise in power and courage; your ideal partner is the one who will both honor and reciprocate it.