Hi Jenrose,
I read your posts and really feel for you as it seems that you really feel strongly about staying home with your children.
Unfortunately you cant FORCE your DP to do anything! So you need to change the things in your life that you are able to change. This is the hardest thing for you to accept, but once you have accepted it you will be able to start making decisions to improve the situation for yourself and your DDs.
Now that you have revealed that you dont in fact live together as yet and you are actually paying for EVERYTHING for yourself and your DDs this is a real concern as you say a large amount of your current expenditure comes from savings.
You dont say whether you are receiving any benefits at all and he really should be paying child support for your baby DD if you are living apart!
Savings dont last forever so you realy need to sort something out before you start getting into debt or you risk falling behind in rent and becoming homeless.
Upthread someone suggested childminding and you dismissed it quite quickly I feel.
You said that your partner likes to spend evenings with you and you feel that childminding paperwork would affect this time together. Well your DP cant have everything his way either! Maybe look into childminding for before/after school/school holidays school-aged children as this might fit around your studies better.
Or maybe you could put both your DDs into a childminder after school hours and do tutoring for primary aged children in that time instead? You wouldnt be away from them for very long each day if you do this.
Try to start thinking logically about it all and planning for the future in a practical way, small steps at first, putting yourself and your DDs as the priority.
If you DP starts taking on an active role in your relationship, divorces his wife, takes on more of an equal role as a partner (and this includes being honest about income/debts etc) then maybe you can consider moving in with him and then you might consider having a 3rd child.
Please dont waste years waiting for him to do what you want him to do!!