Who do I trust, now I think I might not get married ever.

My girlfriend whom we are in a relationship close to a year cheated on me,

BOM I swear I have even purchased an engagement ring awaiting to propose to her on her birthday which is coming up real soon. I could go out of my way to do anything thing for her which she knows. She keeps taking about marriage, her friends getting married, etc while I kept accessing her to be sure I am on the right track. I could condole any faults but not cheating cos I can’t stand people who cheat. If you can cheat during a relationship then there are tendencies you will cheat when you are MARRIED. She made me believe her with full honesty. Lately I began suspecting her but didn’t wanna believe my heart nor show her I don’t trust her. I love this girl and I was really ready to go against all odds just to start a life with her. She isn’t perfect, that I know but I am not a guy who looks around other girls to cheat, I can’t imagine me pants down fucking another girl when my heart isn’t there cos I am gonna get irritated. Every time I talk to her or see her I imagine her fucking the other person even doe she has apologised to me which I felt she wasn’t remorseful. I am lying down feeling sick on drugs and tired of life, I hope I don’t do something really crazy soon. I am taking the ring back to be resold even doe I don’t get paid which I bought with a fortune, I won’t mind as long as I don’t have to see it again.