Welcome to the forum, Babycakes!
I'm sorry to read of your struggles. Brokenness with our children is heartbreaking.
Please know that it's very difficult to offer advice without specific information. Here's what I do during difficult moments with my children - or anyone, for that matter, including people on this forum.
I take a long look at my own responsibility in our upset, apologize, take full ownership for my actions, work out how to not repeat the problem, and we moved forward.
Only when people sincerely understand where they went wrong and apologize for hurting you, can the relationship move forward.
Thinking of your children as "selfish and ungrateful" and yourself as sacrificial who "always put their needs ahead" of yours will get you nowhere. My guess is that they see their upbringing and your actions differently than you do.
Just as there are no perfect children, there are no perfect parents. I'd recommend a sincere effort toward understanding why they've estranged themselves, apologizing for your missteps, and trying to find a way toward peace and a relationship.