Struggling to Get Permission to Go to the Gym (Salam)

Salam. I used to feel and look so much better, but recently people say I look tired, older, and I've put on weight. For months I've been asking my parents to let me exercise or go to the gym, but they keep pushing me to study as if that's everything and I won't survive without it. I'm stuck in my room most of the day and constantly stressed about other things - it's connected to waiting for someone for marriage who I'm now distant from, and since we can't speak because it's not permissible, it hurts and adds to the pressure. I want to feel good, to look healthy, to be active. I'm young and this is the prime time to exercise, but my parents won't allow it. I also want to learn self-defence like MMA so I can protect my future spouse, my mother, my sister, and myself, but all I hear is "study now and you'll have everything later; you have plenty of time." I'm 17 and turning 18 in March - there may be marriage in a few years, or maybe not - but how will I protect my family if I'm weak? They treat studying like the only path to success and reward, and while I understand its importance, sitting in my room studying or doing nothing has left me depressed and feeling worse physically and mentally. What can I do to convince them? I feel like time is slipping away, and my mental well-being depends a lot on my physical health.