Hi baggy:
Unfortunately I don't have the magic wand which you so desperately need. For nothing else could sort this mess out for you, unfortunately.
It's okay the rest of us sitting here saying you should do this or you should do that, you're the one slap bang in the middle of an unwinnable situation.
I mean, walk away and you'll probably regret it for ever and a day. But stay, stick it out and you may also come to hate those closest to you.
I personally believe that it's up to your partner to make a very difficult choice between his son and you, for it sounds as though it's two's company, three's a crowd, as the saying goes.
I'm certainly no expert when it comes to the question of your yearnings for another child, but, have you considered adoption or fostering? I realise it wouldn't be the same as your very own child but you'd still be giving a needy child a home?
The best of luck in whatever you decide to do.