How can this get any worse?

My girlfriend cheated on me last summer with my best friend. They went all the way with it and had sex.

Another bad thing is that she and ALL of my friends knew that they did but knowing had kept it a secret from me. This is like 3 months later and a really good friend of mine asked me if I really wanted to know what happened so I asked him and he told me everything.

After finding out that she had sex with my best friend, made out with him several different times. even more things started unraveling.

She previously told me that one of her friends was going to ask her to go to a school dance and that it was merely as friends and nothing more. So trying to be a "good" girlfriend, she asked for my permission. I said no for good reasons. She went against me and started many fights and pointless arguments, went out and bought a dress for the dance without telling me, and went without my permission really.

It turns out she's been seeing this guy for the last 6 weeks.

This guy is her BOYFRIEND. She hasn't had sex with him but she has done sexual things with him.

She has lied about many situations. After homecoming, I came across a picture of her friend with her date, and my ex-girlfriend and some guy wearing a shirt that matches her dress perfectly. I confronted her about it and she said that she had never met that guy in her life. It was a friend of a friend who just wanted to be in the picture. Turns out it was actually her date/boyfriend.

I've never felt so betrayed in my life. This has been THE hardest situation I've ever dealt with.

Should I take her back? I still love her. We've been together for over 2 years and I can't keep her off my mind. I don't know what to do. She wasn't only being two-faced but tri-faced!

The day that she had sex with my best friend, I actually even dropped her off at his house and even later picked her up from there. THAT IS PAINFUL.

Moments after she got in my car I had a gut feeling and asked "what happened?." and moments later she admitted that she "kissed" my friend. I was willing to be forgiving for that and was ready to forgive her now but it turns out to be way worst than what I could have possibly imagined. I've always believed it was only a kiss. I never expected that they had sex and had made out many other times before. This is terrible.

I have even forgiven my friend for the kiss and was even hanging out with him all the time. I was even his high school senior project mentor.

What do I do? I'm so hurt
How can this get any worse?
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