Wa alaikum salam wa rahmatullah
Depression is because you do not do what you were meant to do.
"And I did not create the jinn and mankind except to worship Me
." Qur'an 51:56
So our sole purpose IS TO worship Allah. When you do that, marrying, to eating, etc. becomes secondary. You said you are going to start praying in'sha'Allah, but i can tell you bra it has ALL to do with praying.
Just like you said
For one minute, when they mentioned my name on graduation day in front of around 300 people, I couldn't believe it
. The mayor of the city handed me a gift, and congratulated me.
My point is, I have all of this happiness in my life and alhamdurilla my parents are very happy with me,
You being happy because of people congratulating you is all temporary. Your parents being proud because you were the best of all those students..it is all secondary. Do not make something that is rather secondary, primary, and what is primary, secondary. Fix this issue first, then you will notice that the happiness never ends as you focus on the primary (worshiping Allah - prayer).
Btw, having a girlfriend is forbidden. Mixing is even forbidden.
"And do not approach unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way.
" Qur'an 17:32
Allah does NOT say, do not do unlawful sexual intercourse, He rather says do not APPROACH it. In other words, doing unlawful sexual intercourse, is forbidden, because approaching it is already forbidden.
Talking to a girl if it is NOT necessary for example is forbidden. If you are at work and work related, keep it work related talk and only that. Do not make it personal. Hanging out is also approaching it which is forbidden.