How can I get him to forgive me and trust me again?

In any relationship, honesty is the best solution to any problem. Also, even if you love this guy with all of your heart, you cannot make him love you back. If he truly loves you, and you tell him the truth about everything, he'll believe you and let it go. That's what love is. Trusting one-another completely. As well, however, if you love him, you owe it to him to be totally honest. Once all of this has transpired, if he believes you and lets it go, it was meant to be. If he still questions and doubts, it just isn't meant to be. It's as simple as that. I'm not trying to be harsh, but that is the simple facts of relationships. There is no relationship if there is no trust. It's like this...Building a house, you need a strong foundation to start on. In a relationship, the trust is the foundation, and continued honesty and manogamy is the house that is eventually built on the foundation. Strong trust, strong house. Understand? Best of luck to you both.