I am currently reading this one but it's coloured
http://www.mangago.zone/read-manga/green_boy/
He followed his crush and joined the gym where she was learning boxing but after getting bullied he realised that he needs to be stronger. I like it so far but I dont know if you will like it ╮( ̄▽ ̄)╭
Hmm maybe this one
http://www.mangago.zone/read-manga/appetite/
It's horror though. The male lead is really weak but he gets stronger eventually
Also this one
http://www.mangago.zone/read-manga/winter_woods/
He literally learns how to be a human. I really love this one
At this point I dont know if what I recommend you is exactly what you asked for but here is another one
http://www.mangago.zone/read-manga/diamond_dust/
She is a piano prodigy who from a very young age was restricted from doing things that could harm her hands (because of her parents and mostly her father). Her father dies and she feels more free but there is still something inside her that bothers her resulting her to not be able to play the piano like she used to. She then meets the male lead and she is gradually getting better. This one has the tragedy tag..you might cry at some point (I almost did)
Im looking for mangas that deal with character development whether that be because they were bullied or had to become better. IM LOOKING FOR SELF DEVELOPMENT MANGAS