Im looking for a manga where there is a or more main characters who only trust the protagonist.
like because they had some trauma from the past they cant trust other people anymore or sth like that and only get close to the protagonist. They can loosen up as the series goes on but it should start like that.
I prefer a shounen or seinen with a male lead, decent art and no bl or gl stuff

You could check out bitter virgin, if you haven't already.

i already read bitter virgin, that exactly what im looking for.
any other ?

Narutaru and Dead end I think, but it´s more like they are fond on the main, but you can check them.

________________
I´m NOT an alien!

You could try Men's Kou. Its not like the characters that the main character meets don't trust anyone else but he does seem to have the ability to get them to open up about themselves. I'm not sure its what you're looking for but its a good manga