Hey Mermaid, welcome to the boards, glad you're here. it's helpful for me to read your experience, because i'm going through a somewhat similar situation emotionally right now. each sitch is unique mind you - we're hubbo & wife, our gf lives pretty far away from us so she can't just come over, but I really identified with a lot of your feelings of being scared, jealous, left out, and feeling like a third wheel.
while i can't offer a whole lot of advice, i will offer you this very solid cyber hug - being on the outside, and having behaved in ways you're not particularly proud of because of those feelings is double hard to deal with.
I have some questions for you:
1) are you getting counselling?
2) does your gf give you whole weekends of time anymore? if not, why not?
3) are you two still doing things just the two of you/have some kind of a schedule so she's not always over?
4) what are five things that would make this situation more comfortable for you?
5) do you have the power to stay "stop" for awhile and just have time between the two of you for a couple of weeks to help your heart/mind/soul feel more secure?
Hugs again - this too shall pass, but i know that not much makes it easier right now.