Please don't say "it's hard to do" because that difficulty is all in your mind. You will not stray, if you do not allow yourself to.
I do not think you're unhappy at all with your current relationship. You've been holding on because you love your GF and, sadly, you just won't realize this until she's gone for good.

I think you're not unhappy; you're just insatiable.
You just can't dump the one you have right now every time someone "better" becomes available.
Imagine being the 3rd "victim" of that "friend" of yours. Have you ever wondered why she can't stay put in one relationship? But then again, she's none of your business. Your GF is.
Spare yourself from causing other people pain. If you think your current relationship is a joke, free yourself from it while you still can.