I been with my boyfriend for 9 yrs and I never cheated on him during the time we were together I went to South Carolina to visit family, I'm not a drinker but had a few to many drinks and met a someone and went with him to his house after the bar around 5am and one thing led to another and we had intercourse and trust me I regret every bit of it but I lied to my boyfriend about the whole thing in the beginning and now its been 4 months since this has happened and I have told him the truth about what happened but he still doesn't believe me how can I get him to believe me so we can move forward with our relationship

If you told him what happened, what is it he does not believe?
Does he think it will happen again?
What kind of relationship do you have with a boyfriend of 9 years? That's a long time.
How old are you guys?

I think that the best thing is to try to discuss that with him and have determination to be better and never do that again.

We are not perfect , we do mistakes , and we have to understand each other , and see the good parts in our relations