But what about all the times I've prayed and have been prayed for? What was that for? Nothing? Is praying useless?
It's really easy to come up with fine-sounding words when you're not the one going through it all so please take this in the spirit it's intended.
Praying is about talking to God. Think of a little girl talking to her parents - sometimes she wants to ask for something, sometimes she wants to talk about what frightens her, sometimes she just wants to talk about her day at school or her friends or what she wants to do at the weekend. If she asks for something her parents might say Yes and give her what she wants, they might say No and not give her what she wants, they might say she can have it later, they might give her something else entirely. The crucial thing is that her parents know more than she does about what is good for her.
When I was a child I wanted to use the big knife my Mummy used to cut up food. I was only four years old and didn't understand why she wouldn't let me hold it. She knew what might happen if I did have the big knife and so she wouldn't let me have it. I thought she was awfully mean, I promised to be careful with it but still she wouldn't let me have it. Now, as an adult myself, I know very well why she wouldn't let me have it.
Paul talked of the times he asked God to remove the thorn in his flesh. Three times he asked, and three times God refused. In the end God said "my grace is sufficient for you, for my power is made perfect in weakness", and Paul wrote about how he would boast in his infirmities. He wouldn't boast about the many other things most of us would consider reason to boast, he'd boast where he was weak. I forget the reference off the top of my head, I think it was 2Co 12:9
I can't say why God might choose not to heal you, at least at this time. I can offer the words of a lady I know who is now in her 60s and has been disabled since birth, and seen her body gradually fall apart through her just trying to live. When her legs didn't work and she walked with crutches it put an extra load on her shoulders and now she can't do that, so uses an electric scooter to get around. Needless to say anything requiring even a single step presents a problem for her. She said that God can use her in ways that probably wouldn't work if she had been able-bodied. There are so many things she physically can't do, and yet so many things she can (and does) still do.