Do adult step children ever grow up and accept that mommy and daddy are never getting back together? Do they every really accept their step-mother as anything other than just "dad's wife"?

I have two married adult step children and between the two of them we have six grandkids. I have tried over and over again to reach out to them to get to know them again (dad and I were married once before when kids were little-then divorced for 12 years and married again), 10 years later, I still feel like a complete stranger....doesn't matter how much money I spend on birthdays, Christmas's, baby showers, etc. I somehow find myself always on the outside looking in. Don't misunderstand, they are always polite to me in my presence, but it feels like I don't matter (example: they will call their Dad to ask if they can come use Dad's pool (dad never uses the pool and expressly installed it for me--go figure!). I have NEVER been invited to lunch or shopping with the girls or any of those normal family type events (in 10 years!!!). All those things are done with their Mother and her family. I am NEVER included.

I am really sick of trying and feel like I am simply wasting my time, energy, and money. At what point do I finally say, "enough is enough?" This is crazy!!!! Any advice?