Steep Mountains

Check Bill Alexander Channel on youtube. Search for Diana Andre videos there. She explains about highlights and does "slow motion" to explain how things work.
Highlights are my weak point as well. I get frustrated at them a lot too. My problems comes ( i think) from absence of knowledge of the form of the tree and where light is supposed to be. Also I just pressure myself down to finish and cannot stop and it creates the mess. I am learning how not to be in hurry and savor the process by taking it slow.
I am sure you will succeed just give yourself more practice time and don't be afraid to erase it completely and redo from scratch. I don't know if you have this fear, but I do and it slows down my learning. We are all different here, I am sure that from many valuable advice on the forum, you'll find something that works perfectly for you.
By the way, once i cried after in frustration I destroyed half of the beautiful painting because how frustrated I was with tree highlights. But after that I gathered myself and finished it after wiping the mess and repainting the destroyed half of the painting. :) A lot of cursing was involved :)

Dark trees at the right side below the mountain look really well. If you remember how you did them, this shall be the key to your trees highlights.