Lonely: Been there. Suck it up. Those that don't learn from their mistakes are doomed to repeat them.
Mom's house/Dad's house: Make sure they have everything they need at your house. Yeah, this all sucks. Make it as easy on them as you can.
B'day partys: Of course they want everyone together. My C would tell you that you are doing them a great disservice to keep up the fantasy of one big happy family. It actually hurts them in the long run. She excludes you on her night, but talks you into including her on your nights? Kevin, she's walking all over you.
Sorry to be so blunt, but .......
Everybody hurts. It's part of life. Don't miss the good stuff.
