Dude, I am just exactly high enough to really answer this. Sit tight.
You have shitty mood swings. The weed might or might not help with that, but the one thing we do know is that it does not help enough. Right now it's threatening your closest relationship, and it's going to make it very hard to ever establish a new one, if this one ends.
It's the kind of thing that can just wreck your life. It fucks up jobs and girlfriends and regular friends and, most of all, you. It maybe feels normal to you, but its not.
I have no opinion as to whether the weed hurts or helps. However, I am certain that if you do not take decisive steps, right now, your girl will leave. Stopping smoking is such a step, but it might or might not do much to fix the underlying problem. Finding a shrink, and being candid with them, and doing to what they say is another such step, and honestly, it's probably your best bet. This is not something that you can fix by yourself, or fix by smoking, or it would be fixed now.
That's what this is about.
Your girl is being straight with you about the fact that she cant put up with it for much longer. You've been with her for years so she's gotta be worth something, and she knows you really well and you must trust her about the important things. So listen to her and believe it.
If you don't fix it then it's just shit, forever. You'll make it but you'll just carry all this hurt around in a place where lots of happiness should be. It's awful to live like that.
You don't want to fix it someday, you want to fix it now before your girl is gone from you. She'll help, too. She'll help you through it and it'll come out OK, and you'll thank her the rest of your life.
Choose the right thing, and go.