Hello

Im a Heads Up player. I thing that one of my biggest leaks is bad understanding of how to construct a 3bet range.
Now I want to fix it, but I do not know how.
I want to know - WHY, why this hand is good or bad to 3bet vs this opponent? etc
Because right now my 3bet range is a total MESS.

Any videos , other resources? Generally how I should work to improve this? This is most important for me because I want to start working on it off the tables, just i dont know how.