How to gain his trust back?

We have been seeing each other for about a month now. Neither one of us have made that commitment to one another yet. However I have really strong feelings for him and can't fathom the thought of losing him. The other day he asked me a question about my past. He asked if I had slept with someone that fit this particular category and I told him no. Later that day I called him back and told him that I did. Now I lied to him and he doesn't trust me at all. Even though I turned around and told him the same day. Not everyone is proud of their past and maybe I was just a little apprehensive about telling him. I'm not a good liar at all. He questions everything I do even my feelings for him. I want his trust back because we were great together before this happened. Now he cant even commit to me. He believes everything I say is a lie. I'm not even sure if I'll be able to gain his trust back. I really would like some input..Please!