/improve/ - Self Improvement

I discovered what's wrong with me, but I have no clue about how to solve it.

Imagine an average footballer (soccer) in an average team. Not a gifted player, just an average. His career starts at youth level, 17-18 years. Around 20 he's called to the first team, at 21-25 he improves, gets more minutes in play and by 27-28 he's a regular in the starting line. He's certainly not Messi, but he's decent and reliable, and plays regularly until his level drops by age, contributing with experience in his late years and retiring between 35-40. That's a "normal" footballer career.

Now imagine a footballer that gets to first team at 20 too. He has a promising season, but the next year his skills start to drop quickly. He tries his best, however for some reason he already reached his peak and now he's decaying. By 22 he barely gets to play, and he has no choice but retiring at 23.

That has been my life, for anything I try to do. I start good as a beginner, but then I just don't improve, like I never "level up". And then I fall until I'm the worst, get frustrated and I can't continue. It's not about lack of motivation and slacking off… no. I try but then I can't anymore. It's weird. They say "practice makes perfect", but it's like for me, practice stops working at some point and makes me worse.

I had to abandon college, because eventually I failed every exam no matter how much I studied. I was fired from two jobs because I wasn't working as good as before. I started to do a bit of exercise (because everyone tells wonders about excercise), walking a long stretch everyday after work and riding bike on weekends, but after some months I saw no progress and I was puking after each bike ride. I even stopped playing some videogames because my performance was dropping and I was getting frustrated.

I feel like everything is in my head. I stop trusting on myself and never pass the "beginner" stage on whatever I do. And now, I have no idea of what to do. Am I doomed to never achieve any goal?