Advice please, I turn to you ladies because I value your opinion

So I suspect my SO has been lying to me for some quite time now . In 2015 we had our own place and been together since 2010 we have two children together. So a female had told me on social media that her and my SO been dating for the passed six months and she's pregnant and blah blah blah. This girl was texting and calling like crazy and she has been around my kids without my knowledge when I was at work. My SO was nder her phone line and my SO let her borrow his car and found a CVS receipt of pregnancy test left in his car which wasn't mine, let me tell you this much idk her I only know her name . My SO has told me she had a thing for him and that he told her he was "taken" it's still in my head after 2 years and eating me alive that I have no proof other than what she told me. Idk who to believe she was crazy tried to fight me while she was "pregnant" nor was there any kid born . My tires got slashed all 4 of them on January 20,2016 shortly after all this drama. No one knew where we lived because I never invited any friends or family over neither has he only a few people and she has also been there . Please help I'm going crazy , I've tried to talk to my SO about it and he denies having a thing with her. I don't like to bring up previous stuff that is done and over with and I can't get it out of my head. I keep replaying everything that has happend and I really wanna believe my SO but idk . Most of you will say if you don't trust him you should leave . I don't want to leave if he is really telling the truth. Of course you guys don't know me or my SO but I would like you to tell me what you think is he lying or she or both or what!!!!

Edit: she asked to borrow his car and he let her borrow it he claims he lent it to her to pick his sister up from work but I don't believe him . I have dug and looked but nothing everyone I talked to says she was crazy and all this and that he had done nothing with her . These people have no reason to cover for him they weren't his true friends they have done him dirty. Please I think about it everyday.He no longer talks to this person but I relive this everyday.