His Six Years Relationship Is Falling Apart, Please Advice

I've been dating her for six years. 3 years ago i found out she was seeing someone else, but when i asked her she said she was going to stop seeing the guy immediately but till date she still goes to see the guy, sometimes even in the guy's school.I am in my final year while she is in her 3rd year. The other guy is in his OND1. i feel betrayed and wanna end the relationship but all her friends are begging me to give her time to put an end to the affair with the guy. She also begged me to pardon her that she will end it. My problem is i can't share my girl with someone else especially in this era of STDs and even HIV. I love her and anytime i ask her to stay away from me i fall sick after a day or two. Please guys and babes, i need sincere and genuine advice. I don't wanna act and regret later.I am confused that's why i brought it to the house for good advice. Please pals help a bro that is helpless now
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