You sound like you are open to sensible advice. The problem here is that you are not in love with this woman any more. The rule re relationships goes like this: you will get more of the same. Why? Because people can't change.
No woman who loves you would isolate you from your friends and family. Ending a relationship like yours is never easy. However, you really aren't obligated to look after her children. Of course they will miss you. Not sure if it is good to try to keep seeing them.
She doesn't sound like a reasonable woman. If she were you wouldn't have this decision to make. Eventually you will have no choice but to leave so the question is should it be sooner or later?
To thine own self be true. Sometimes hard decisions need to be made and it's painful.
Either that, or stay with her and be a fucking man. Tell her how it's going to be (firmly but nicely). If you are providing for them and giving them a good life, you need to let her know that you have needs too and that you have a right to expect some things of her. Women can be selfish as fuck sometimes. If you assert yourself in the right way you might be surprised in her reaction. Women need a man to be in charge even if they don't know it. You've probably been treating her so good that she lost respect for you. Women will run you around if you let them. Be the man of the house....just make sure to be a righteous man.
As long as you aren't doing wrong like cheating on her or beating her her ass, she needs to show some appreciation and care about your wants and needs too. Respect is a two way street.
These 2 answers are your best guide.
Speak to her Explain your issues & tell her how she needs to adjust
If she loves you & cares anything for her kids and what you have said is
true, if she's a reasonably intelligent woman she should be able to understand
& make changes.
Perhaps she hasn't got over the death of her husband
& may need help.
Its good to hear you have a loving caring relationship with her kids
Clearly its a tough decision If she will not understand & change.
Good Luck.