Stepson's b'day, and a hard thing to deal with

Today is my stepson's 7th birthday, the first one in 4 years that I wont be there for.

For the last couple, we had a great time planning and making the cake - I haven't seen him since the night his father and I split. By now I know I've mentioned that our D is because of abuse, the last incident in particular being the worst event leading to STBXH's arrest and my hospitalization. sS was there for the whole thing, and one thing I regret is that I wasn't able to shield him more. I should have sent him upstairs sooner, I should have told him to run and call the police instead of trying to do it all myself.
Another part I regret is that this small person, that I loved independently of STBXH, can no longer be in my life because I'm terminating my marriage to his father. I have never had much contact with sS's BM, STBXH's first wife. Right after the "big bang", as I call it, we had minimal correspondence before she basically told me to leave her out of it, she'd already been through one hella divorce with STBXH and didn't want to be again. I see her point and although I wouldn't want to involve her at all, I don't want her as an enemy. But this leaves me not talking to sS's mother or father and basically... never seeing this little boy again, and I'd come to imagine myself being there for the rest of his life.

If anyone has ever been in either role - mine, or sS's mother, do you have any words of advice? In any case, it's helpful to just...type it out loud, I guess.