How can I learn to trust him again?

If he doesn't understand, you need to make him understand that you have been hurt by him and are willing to give him a chance, but it will take some time before you can feel so comfortable with him again. Let him know that you want to make things work, but you will need his help in being honest with you and being truthful and trustworthy, and he needs to understand that you are just trying to protect yourself from getting hurt. If he really cares, he'll understand. You have every right to have reservations, and even keep your guards up. Protect yourself and your feelings girl. And when the time is right, and you both start becoming comfortable again, then awesome. But in the mean time, you need to put yourself first before you get hurt again. Good luck, and I hope it all works out for you.