With what you posted so far either you are not following what your dietitians told you to do or they are just idiots.
You have to understand that they may not understand your goals. You are healthy for a long distance marathon runner or biker but for someone who wants some size you are not. Some dietitians just can not get that through there heads. They think less is more.
You have spent years trying the wrong things. Maybe not your fault but it was still wrong. Why not spend a little time trying the right things?
To old? I learned how to eat FINALLY when I turned 45. I wish I found out 20+ years earlier but it's never to late. I am in better shape now than I was at 25. See my avatar? No it's not me yet... I have another year, 2 at the max and it will be but Im really not that far off. Here is me now, almost 48.
Yeah I still have work to do, that's why I said another year at least.