Boyfriend troubles

So my boyfriend cheated on me with his ex. He said it was just a kiss and that he will never do it to me again cus he loves me. And I love him with all my heart and I just don’t know if I’m stupid for believing him when he says it won’t happen again. But now I just feel like his ex is a threat to me and that in competition with her and I hate feeling like that and I hate feeling insecure and I just don’t know what to do