You already said you don't want children. You're taking up her precious time by sitting on the pot here. You need to give her a chance to find her strength and find a man who will make her happy who does want children with her. Her clock is ticking and you're not helping.

Let's say you're being totally honest and open here. If that's the case, if all these people have convinced you that this woman is on thin ice and her life won't go on without you, then they're all manipulating you--they're all emotionally abusing you--to stay with her. That's so unhealthy. You deserve better than to have a selfless life trying to preserve this one other precarious life. And she deserves better than this sham life built on pity. Trust me, I've played both parts in relationships in the past, neither scenarios made me happy. And I recovered from both once I got to being alone and sought therapy.

Now, let's say you're not being honest and are trying to manipulate all of us into absolving your guilt because you're such a nice guy and it's not your fault and blah blah blah. You're not a nice guy. Go re-read my first comment before it sinks in. Nice guys can feel stuck, nice guys can develop feelings for others, nice guys can desire to feel needed, but nice guys don't drag on a relationship with so many fundamental points of disagreement while having multiple affairs on the side.

Either way, you, your girlfriend, and probably the two other 'loves of your life' all need to be single, get therapy alone, and and figure out how to treat yourselves and others before re-entering the dating pool. You aren't responsible for anyone's happiness but your own and you don't owe it to anyone to hold them together. You owe it to people to trust that they'll pull themselves up by the bootstraps once it's clear they're their only keeper.