How do I break my girlfriend's heart?

My girlfriend and I have been dating for several months now, and unfortunately things are perfectly fine. I say unfortunately because my ex and I just admitted to each other that we still love each other. Not only that, we both made it clear that neither of us are happy in our respective relationships. Us ending up together seems more like an inevitability than a possibility. So then how do I throw away the perfectly healthy relationship I already have? I have no outstanding issues with my current girlfriend, but I also don't feel extremely attracted to her like I do my ex. This is exacerbated by the fact that the relationship is long distance. She is extremely intelligent and caring, but I'm just not feeling it. Staying in a relationship that I'm not emotionally invested in is unhealthy, disingenuous, and unfair to anyone involved, which is why I've come to the conclusion that I want to break up with her.
To further complicate things, both of our significant others are integral parts of our collective friend group. Not only would our breaking up with our respective significant others be damaging to our group as a whole, but we would more than likely (given ample time to heal and make sure it is what we both truly want of course) end up together. If our significant others are still a part of the friend group after the relationships desolve, and then we get back together, it would undoubtedly damage things even further.
I know that honesty is the best course of action, but there are so many moving pieces in this conundrum that makes the whole thing seem impossible. My ex and I decided that we were going to give each other enough time and space to get our heads on straight in order to not make any rash decisions, but putting this off leaves a bad taste in my mouth. I hate not telling my s/o something so important, even if it would hurt her to no end. It just isn't right.
So what then should I do? Wait, tell her and break up, try to bury my honest feelings for my ex, tell her and not break up to see where things go? I'm at a loss.