Well, I'm unfortunately speaking from experience here and the main point is this.
With a guy, at least with me, actually caring for someone, or god forbid, loving someone takes time. And if we are sure we do love that girl, and we know she loves us, its all good, and she can have our trust. When that is broken however, ANY girl that comes after that is going to come with serious doubts, and any and all defense mechanisms come out, and all the emotional walls go up.
The only way a girl is going to be able to earn the trust of a guy like that is to make yourself such a part of his life that he becomes sick of you, cause only then can he love you.
Sounds stupid, I know, but the point is this, a guy will let down walls if he is comfortable and whatnot, and tust can develop with time, but it will take time, because any smart guy won't let himself get into a situation where there are tell tale signs that something is likely to go wrong in the future. (Its amazing how adept people become at this after they've been in bad relationships before) But, if you're persistent, and let him know you care, he'll come around. If not, he won't ever, no matter how much you show you care.