relationship problem

so me and my boyfriend have been together for almost 2 years and he always accuses me of cheating and I have never cheated. also, I have caught him hitting up girls and flirting and always give me another chance and he constantly goes out every weekend. and recently, this one girl contacted me sent me screen shots of him talking to her and she is going to send me more, but he told me that she was trying to get with him, but the screen shots say other wise. I don't know what to do, I love him with everything I have & this is really hard for me.