<human>: So I've cheated on this guy twice. Yeah, I'm that girl. The first time was not in-person-physical, just text and phone-based. After that ended, my relationship actually got stronger and much better. Things got much worse towards the end of the semester because of stress. After I convinced myself that he and I were not going to make it as a couple, I got physically and emotionally involved with a very close friend of mine (actually because I had feelings for him, not just to fill the gap my boyfriend left behind). I didn't break up with my boyfriend before starting this because I didn't want to cause him stress before his exams. 

Well, shortly after exams, he became the sweetest, most caring boyfriend in the world and has put all of his focus on me. I know my heart doesn't love all of him anymore, or I wouldn't be so into another guy. So I eventually mustered up enough courage to break up with him tonight, but I didn't tell him that I'd been seeing someone else. I loved my boyfriend very very much when things were good and I just can't hurt him like that. We're both destroyed right now, but he wants to continue to talk because he wants me to fix the way I see him, which is what I told him was the problem this evening. I know he doesn't want to let me go, and his being so perfect after exams really threw me for a loop because that's the version of him I wanted all along. For the record, we had been together for ~nine months. 

How can I be sure he isn't right for me after all when I thought he would never change and he suddenly is the perfect guy? And should I keep him in the dark about the other guy and just blame it all on me so he doesn't get hurt?

Condense the text into a few key points
<bot>: Dated guy for nine months, cheated on him twice, know I shouldn't stay with him, but should I tell him about my infidelity?