I grieve for the parenting experience and the children we are told we will have.
Mine have SEN and one has a difficult mental condition caused by trauma she experienced; it's permanently changed her brain. Life is going to be hard on her.
I try and count my blessings every day. But many days, I feel awful for having brought them into this world, without all the abilities they should have.
I understand you. And what you've experienced and are experiencing is more than we have to go through at my house. It takes a strong person to look after someone like your son, and you've done it for 25 years now.
It's not wrong to wonder how life might have been. It's normal. What would be positive is writing down a bucket list of things you'd like to do or experience, no matter how big or small, and see how many you can tick off. You'll be surprised.
If it takes it, start a crowdfunder/ gofundme and go on an adventure, with or without your son.
You deserve some happiness.