Active imagination?

My boyfriend and I have been dating on and off for over a year, and this time it's been almost 4 months. But its a long distance relationship..about 2 hours away. And I don't know if he has ever cheated on me, but i get so paranoid all the time and I can't help it. I just hate that feeling of if he ever cheated on me, that other girl would feel like she has one over on me, like she can be with MY boyfriend. Like I said..this is the paranoid me. But he told me that if he ever cheated on me he probably wouldn't tell me because he wouldn't wanna ruin our relationship. Well, then how do I ever know??? It sucks to feel like there could be another girl who thinks I'm a total idiot cause I'm dating someone who is cheating on me. Worst feeling ever. And I guess it wouldnt be so bad if he cheated and didnt tell me and learned from it and regretted it..but what if he realizes he can get away with it and does it more? Then I feel like I'm not good enough and there's someone out there better than me for him that hes keeping from me and making me feel like a dumb idiot. I guess my tip would be...just dont jump to conclusions about anything especially if you dont have any evidence. Don't accuse if you have no reason. It's just my active imagination I guess