I'm not saying you shouldn't do things for the benefit of your kids or that your needs trump theirs. But your health and your needs are important, too. You need to have interests and relationships that are healthy and fulfilling for you, and they should be because you want them.
According to my wife, my MIL managed to isolate herself from all of her friends over the years because she didn't make time for them when they wanted to do things with her. There were always sports practices, church activities, or homework to help the kids with. I can say from experience: if you keep being told that someone is too busy to spend time with you, you will eventually stop asking because if they keep telling you over and over again that they're too busy, they're telling you that they do not value that relationship.
My wife says my MIL becomes a different person the rare times she manages to reconnect with old friends (who live on the other side of the country) at weddings or other major events; she's happy and relaxed. Now that all of her kids are out of the house, she doesn't know what to do with herself. Heck, she and my FIL could actually move to be closer to these friends and to get away from extended family drama, but she chooses not to for some reason.
Looking back, I see that you did say that it's important for you in future relationships as well as for your kids. So I'm sorry if I came across as more antagonistic than I intended.
Me: 45, Her: 40. Married 16 1/2 years, together(-ish) 20.
Status: BD 8/25/09, she moved out 8/28/10. No talk of D.
Every day is another chance to get it right.
http://www.vachss.com/mission/behavior.html"Counting days won't buy us years" —Wings by HAERTS
"Forgiveness means giving up all hope for a better past." —Lily Tomlin
"When we commit to our lovers, we implicitly promise to forgive them. There is no other way we can live with someone for better or worse or until death do us part." —Dr. Frederic Luskin