Puer Aeternus and My Experience

I’m a 21-year-old university student. For years, I’ve been the kind of person who focuses intensely on something for 4–5 weeks and then gives up. Every time I quit, I feel like a failure and fall into a depressive state. I’ll go on a diet for a month, lose 5 kilos, then gain it all back the next month — and this has happened maybe twenty times. Yet every time, I still think, “This time it’ll work.” Because I believe I have potential.
And honestly, I do — if I just kept going for 3 months, I’d have an amazing physique. If I studied Java for 3 months, I’d make huge progress. But I don’t.

Until university, I was a bright student — the kind teachers liked and said would “become a great man one day.” Relatives had high hopes for me. But things didn’t go that way. I ended up in a mediocre university, studying computer science. I chose this field because even if the university was bad, I thought I could unlock my potential here. But now I’m in my third year, and nothing has changed — bad GPA, zero experience.

I always blamed my surroundings. I moved into a dorm — didn’t help. I rented a student apartment — still didn’t help. Eventually, I realized the problem is me.
But the thing is, I just don’t fit anywhere. When I first rented the apartment, I was so stressed I almost cried — the rent, the bills, the cooking… I told myself, “If I get a chance, I’ll cancel the contract.”
Now that chance has come — but I don’t want to leave, and I don’t want to stay either. I feel like I belong nowhere.

These feelings perfectly describe the Puer aeternus. I’m aware of it, and I’m trying to resist it. I take long walks by the sea and end up crying sessions.

My question is: how do I become a great man?