We are both in our late 20s and been together for 3 years. When we started seeing each other she used to only do open relationships, she was clear about that at the beginning. I didn't want that, but we talked about and agreed we would set up rules and only allow her to see other people if we came to an agreement about it.

I don't have a cuck fantasy, but accepted it as I really liked her and it was the only way she was willing to get into a relationship.

On the first 2 and a half years we ended up being monogamous and I completely forgot about this whole conversation about her seeing other people.

About 2 months ago she told me that she was out and met a guy at a bar and she didn't do anything but she wanted to meet him a have sex with him. She reminded me of the agreement we made at the beginning of hour relationship and all that. I didn't take it well and was like absolutely not. It ended up in an argument and she took it very badly.

The truth is I'm crazy about her and after several days of talking about this, we ended up agreeing on her seeing him with several rules.

Rules: I don't wanna meet him. Must use condom. No anal. No cuming in her mouth or face. And absolutely no one that we know can find out about this. As I find it very embarrassing.

She agreed and went to meet him the Friday evening of that week. Since then, they started seeing each other once a week. It's painful to me, but she didn't change at all the way she act towards me, so I learned to accept it.

Last week she told me he was organising a party and she is going with him. The party was this past Friday. I agreed as long as no one that knows us will be seeing them.

I came to find out last night that the party was a gangbang party and that he organises parties like this every month.

I immediately freaked out, started a massive argument and asked a lot of questions to which I really wasn't prepared to hear the answers.

Basically was a party where it was her, another girl and several men.

We haven't spoken since last night. I told her horrible things that I probably shouldn't, but I feel very betrayed and not sure l'll be able to continue with this relationship.

She is the woman I was thinking to propose and now I don't know how to even look her in the face.

How do I fix this?