Re: I'm a girl-i lost his trust and i want to win it back!
While Flyer's post is good and provides the prototypical advice on how to get your ex back, I'm sort of curious if you're even at that stage yet. I mean, have you tried to just talk to him about it? Have you told him why you didn't tell him and that you made a mistake you won't let happen again?
If you haven't done this it is the first step before you go into full-blown "get my ex back" mode. Just make sure that you're not overly emotional about it, but a little emotion can go a long way. Men have different triggers than women when it comes to this stuff. Women want an alpha male, so when there is a break up, the best thing a man can do to get his ex back is act unaffected. Men on the other hand, are very possessive and protective--even of exes. Men typically have a desire to "make everything better," so showing that you are negatively affected might actually make him try to "fix" the problem.
If that doesn't work (I believe it's the route you should try first), then start playing the jealousy card. Start hitting on other guys around him and making yourself very available when he is around. If he's like other guys (I.e., he has a penis), this will drive him mad and he'll either try to get you back or get pissed off. Either one is good because both mean you've had an impact on his emotional state, which is what you're looking for.
"It's only after we've lost everything that we're free to do anything."