Been with my girlfriend 3 years, love her, but bored of the relationship

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Been with my girlfriend 3 years, love her, but bored of the relationship and in all honesty would rather be single but I don't want to break her heart. Especially as she has no idea I feel this way, from her POV everything is perfect. We don't live together but she sleeps over 3/4 days a week.

3 weeks ago we both met this girl who has come from overseas for a year on an exchange, I'll call her X. She got with a close friend of mine (Y) on the first night we all met at a party, they then had sex a couple more times, then she shut that shit down. This was after she came on to me earlier in the night and I shut her down because I have a gf (not to mention my gf was there) - my gf saw her come onto me, my gf hates X with a passion.

Last night I was at a party again, gf and close friend were not there, X was there and she came up to me and was being very touchy feely and coming onto me hard and I pushed her away, we still spoke later in the night and she was saying all sorts of shit like "I would have got with you over Y that first night if you didn't have a girlfriend" and she again was being touchy feely and I pushed her away. Another close friend of mine (Z) and my girlfriends saw and heard all this. Later on in the night X was extremely drunk and I was also extremely drunk, I sent X a message making sure she got home okay. She said yes. That was the entirety of that conversation. I have not told my gf about this night.

This Friday there is another party. Me, my girlfriend, X, Y and Z are all going to be there and I have a great fear that a whole load of shit is gonna explode in my face despite the fact I don't think I've really done anything too wrong...

Bonus Info: I do not like and am not attracted to X. But I also kinda want to break up with my gf. But there's now no way I can break up with her without making it look to her and everyone that I'm breaking up with her as a result of X's advances.

WHAT DO I DO