I get it. I am one if them too. I understand your feeling.

You want love but keep failing. Even though, you were really trying your best. You were there when they needed you. You put effort.

You have to zoom out and understand what is happening.

Lets start at the foundation of the problem.
Every nice guy is a nice guy because in his life there was one or multiple moments when it wasn't safe or comfortable to be himself.

For example: for me one of these moments was being bullied and picked on by kids in school.

For seduction this is fatal. This has 2 reasons.
Practically being a niceguy doesn't seduce women. It pushes them away.
More important, i know you don't want to be this way. For you mentally it's hard to be this way. In essence the problem is anxiety.

If you want to get rid of this, There's a book called: "No more mr nice guy". I'd really recommend it.