How can i regain my boyfriend's trust back?

Understand that we are not commenting on your cheating, because your relationships and sex life are your business. However, your boyfriend has absolutely no reason to trust you now or later. Whether or not you can regain his trust rests principally on how gullible he is, and/or on how strongly he feels about the situation to begin with.

That said, a nice apology and promise, then doing absolutely nothing that could raise his suspicions again, would seem the only practical way. If you can't stop cheating, you owe it to yourself and to him to tell the truth and let him decide about the relationship.