She puts herself and career ahead of her children and always has done. I am a totally different person and give up my Carer the moment I had my first child because I felt that was better and went back part time once they went to school full time.
If I understand correctly, your boyfriend has done exactly the same as her - put himself and his career ahead of his children. Why the scathing indictment of her and not him?
Really, your issues are with your BF. The only thing the Ex-wife has done is to ask if you would do day a week extra, and offered her BF to do a day a week extra in exchange.
That seems quite reasonable. You could have said 'no', but you were happy to do it in the beginning. The problem is that it never got reciprocated - has your BF been pushing for Ex's bf to take them, or just forgotten about it because you were doing it and that's the path of least resistance?
You are completely allowed to say that you're exhausted, and will no longer do any additional childcare. You need a break. It's ex-wife's bf's turn. And if he won't do it, they'll have to figure out a different arrangement between themselves.
Why has your BF allowed his Ex to change weekends on him this weekend? Again, path of least resistance? Or did you not tell him you wanted some time off? I suggest you tell him that you're going to be having the weekend off regardless, and he can do the rest of the childcare (after your lovely bike ride), and cooking their meals etc etc. You can be Disney stepmum for a change :)
In summary - start prioritising what you want/need. Let BF know - he's not a mindreader. Draw your own boundaries and hold firm. It's his responsibility to deal with the children when it's his time, not his Ex's, and not yours.