Wow. I hate to say 'he saw you coming' - and I'm sure it's really not at all as simple as that. But...!
If you looked at this with a very jaded eye you could easily see - a very typically slightly entitled and lazy man who finds himself suddenly genuinely solely responsible for his children both financially and practically post divorce. He doesn't like this at all... and so promptly finds a nicely solvent replacement model, cements his position by having a child with her and creating said family unit, then sits back and lets her take on the lion's share of both the mental and physical load for HIS kids as well as their joint one. With this guy... he isn't even bringing in much cash, so the freeloading is also financial. I can't believe you're taking responsibility for his childrens' school uniforms. Omg.
So that's the very cynical viewpoint. But what's there to disprove it? Even if the situation that's developed is unintentional, the fact that this is how it is - you paying, you cleaning, you running around - has corroded things. Which is not surprising- a woman would have to be a real doormat to close her eyes to the cheekiness of this situation for long. Your eyes are open, and you're not here saying, but I love him, but he really supports me, but he genuinely puts me first and takes so much of the stress on his shoulders... because he doesn't.
(Needless to say if this was his plan all along that's a whole other level of parasite - I guess only you have your inklings on that).
The thing is, you aren't going to be able to choose to continue to put your own child second, throughout his childhood, to bankroll and shove up for this taker and HIS children. Smaller room. Less money. A bunch of bigger half-siblings and all that comes along with that. A more stressed mum. Less time less money less peace.
You resent your DP for this now and you bloody should - he's taking you for a mug, even if only because he's lazy and selfish.
I would split, absolutely, in your case - especially as on top of everything else, it's your home, you have your security, you just don't need to keep doing this. And you won't stop resenting him now - it'll just snowball.