I am looking for a a shounen/seinen manga where the male lead tries to win a girl over and eventually does but with alot of problems in between for example starts to like another girl or the girl starts to like another guy. Examples would be Love Hina , Ginen Shounen
What you are describing is pretty much the classic plot for most shounen/seinen romance. Though a lot of shounen/seinen romance tend to focus on a love triangle rather than one directional love.
A bit more on the adult side, I would recommend Hetakoi. It is a bit smutty and you are warned in advance there is nudity and sex involved, but probably my favorite of these kinds of romances.
I"s is probably my favorite shounen romance. With Video Girl Ai by the same author being very close as well. Both still focus primarily on one girl though. But, both suffer from some of the shounen faults of being very idealist and focusing on the main character being indecisive or unsure of himself.
Bitter Virgin has several other themes involved. But is also a very good read.