My girl friend dumped me because of something I did in the past

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The thing is I'm not sure why it happened it just did. I was happly with her since high school and we've been together for about 5 months after that when my friend starts telling me she's cheating on me. She always had something else to do when I ask her out and she couldn't stay out late. It wasn't till probably a week later when I was at our local mall that I saw her with another guy. I was crushed and angry at her for what she did. I grabed her hand and tried pulling her away from the guy to talk with her alone but he steped in front of me snatching her hand away. I wanted to punch him but I couldn't bring myself to do it because of her standing behind him. my friend was right the whole time and I didn't trust him. I'm still single afraid the same thing might happen again. It's been this way since last year. hwat should I do?