One of the biggest challenges in dealing with those who are estranged from you is the their occasional contact to simply ask you for something. It's especially difficult when those people are your children. As parents, we naturally want to do all we can for our kids, no matter what. We want them to be happy, taken care of, have what they need, etc. So, when they come to us after days/weeks/months of silence, it's easy to want to drop everything and help them out.
The problem with that is that you're teaching them that your well being, your hurt, your emotions and feelings aren't to be taken seriously.
You teach others how you want to be treated. If you revert to being a doormat, you'll be treated as one.
Standing up for yourself may be hard, may cause some anger and hurt feelings, but in the long run is one of the best things you can do for yourself and for them.
For more, check out the Estranged series on YouTube