I don’t see any problem with his ex, she’s not getting the money, his kids are. She also has the children living with her all the time they are not at uni - that’s 22 - 28 weeks a year ( and even more this year). Teenagers are expensive to feed, clothe and entertain.
It’s entirely reasonable that he supports his children through university - the student loans are calculated on the basis of parents’ earnings.
The only thing that’s unreasonable is that he’s doing all this with your money. Which, as I understand it, you agreed to for 6 months but you have gone on doing it for three years.
I think your anger at his kids and their mother is entirely misplaced. You should be angry at him for taking advantage of your good intentions. And angry at yourself for letting this go for for an extra 2.5 years.
It’s up to you if you try to recover the money you have poured into his business start up. I suspect you will lose it all - these ventures are notoriously risky. If you are smart enough to be a high earner then you knew this.
Even if his business was successful it’s still very hard to get your investment out, unless you have a buyer. I’m assuming that you have shares in his company ?
I think, like many of us do, you have made risky decisions for love and you now regret it. But that was your choice - it doesn’t seem as if he’s been dishonest in any way.
Some people might think it was irresponsible of him to fail to earn enough to provide for his children and I’d agree with them. But again you agreed to his plans and you must have known that very few businesses make a lot of money in 6 months.
Of course you are absolutely entitled to now draw a line under it and give him notice that you will be stopping your subsidy to his business in x months. I suspect there will be fall out and that will tell you a lot about what kind of man he is.
I hope you can both work this out.