ok so I tried to find this yaoi but couldn't.. the only thing I remember is that it was a lelationship between a college/university student with longer hair and an older man....

someone an idea?

I really need more details. If I search age gap, student/s, older seme younger uke, I get a lot of names.
If it's on your lists, why don't you try doing a advanced search?
Ai ga Kami wo Korosu Toki, for example, could fit your request, but I need more details.
Was it romantic? Is the hair is really long? Supernatural? Mafia? Dark ambience? Fluffy?
I'm also curious!

well the problem is I don't remember much...but the hair of the college student has a lighter color and there was this extra at the end of the manga were the college student sits together with his friends and they tell him to get a girlfriend and he thinks I already have a lover....something like this..the extra was on one page with a lot of small frames

the hair is not really long more shoulder length and no supernatural or fantasy....
maybe art and feeling of the manga similar to FUJIYAMA Hyouta but it's none of hers.

Castle Mango maybe? This fits to many points you've made.

no its not castle mango 🙂