Well if you want to gain elo big time, it's easy, just don't play lee.
He's one of the highest skill cap champ tbh, and after 30min you literally transform into a potatoe.
(at least if you don't have kills and/or can insec like a boss)
If you're behind on lee you can just hope to get carried because there is no way you will
also if sidelanes lose you're insta fucked.
I would highly suggest bruiser meta junglers like xin ww etc to carry, or learn to play evelynn/khazix/sylas.
Jokes aside, as playing lee, you should focus on dunking one lane particularly, pick the most cc efficient one and spend your life there.
Just make them tilt, that's all it's about until very high elo.