Is this normal?

I've been with my boyfriend for about three months almost four. I am 20 and this has been my longest relationship. My second one, too. I wish I could stop, but my head is always plagued with thoughts of cheating on him before he cheats on me.

Even if he hasn't given me a reason to doubt me, he always shows it, tells me he is committed, I can really see his devotion towards me, but I still can't change these thoughts and I'm scared I'll act upon them because I almost did once..

I mean, every guy cheats right?