OP, you sound exhausted.
Working takes energy, doing work you don't enjoy takes orders of magnitude more energy, studying takes energy, studying while tired takes energy you don't even have, basic adulting takes energy, having fun takes energy, and dealing with a chronic illness takes an ENORMOUS amount of energy.
Something has to give before you will have the capacity to really enjoy life. You need a surplus of psychological and/or physical energy in order to be mentally healthy and happy.
Living in survival mode/ "reserve mode' just isn't fun, and there's really no way to make it fun either. Being physically and emotionally perpetually overdrawn is simply toxic.
A balanced life involves a combination of activities that simultaneously drain and generate energy. A long hard day of challenging work partially drains physical and focus energy, but fuels the satisfaction and excitement energies. However, if you enter into a long and hard work day with too little physical and focus energy, you can't recharge anything and operate only on "reserve mode".
In "reserve mode" you never recharge any energy. This makes all "down time" reserved for sitting the fuck down and doing nothing, which replenishes the baseline energy reserves just enough to maintain "reserve mode", but never lets you reenergize enough to get back up to full capacity. It's why hobbies and socializing are draining and feel like work.
It's like always using your phone in "power save" mode where you never actually get to use your phone's best features. Sure, you can make a call, but what's a smart phone without the capacity to stream cat videos??
As a result, you are living a hollow shell of a life never getting to experience the world in its full colour. You are living perpetually in grey-scale. You can't even know what would make you happy because you've never experienced having enough capacity to ever be overall happy.
We have this bizarre martyr culture where when we start failing due to lack of internal and external resources, we instinctively push ourselves harder, as if a lack of resources is a personal failing and not just a reality of our personal limitations.
It's like someone who runs a marathon being disappointed with their time because they weren't rested enough, so they immediately start running it again right then and there under the premise that if they just try harder that they will go faster, even though they're infinitely more exhausted than when they woke up.
It's illogical. Yet, it's what most people instinctively do.
You look at your lower grades and think of how you should "punish" yourself with harsher study practices. Meanwhile, knowing that you are perfectly capable of performing better, you could look at those grades as a warning sign that you are using up too much of your available resources, and that pushing yourself harder is actually the least efficient option available.
If a shortage of resources is the problem, then aiming to drain even more of those precious resources without cutting back somewhere else is probably the worst move.
You're not wrong, you really aren't living and you can feel it. You are surviving. Survival mode is not living, it's the psychological equivalent of eating only meal replacement shakes. Sure, you will survive, but you aren't eating and enjoying food the way others do. It's not the same experience.
Personally, I consider living in survival mode to be a state of emergency. I think knuckling-down is the worst possible response in that state unless it's absolutely necessary for your literal survival.
Just like financial debt is a hair-on-fire situation, energy debt is an even bigger HAIR-ON-FUCKING-FIRE situation.
Practicing gratitude is one thing, but being complicit in your own self torture is another. If you have any other options available to you to start living a better life, then being grateful for a suboptimal life is nonsense.
Aiming to maintain this state for 12 long years until you can FIRE at 40 sounds insane and self destructive to me. I recommend looking at your life right now and seeing where you can make more space for yourself.
Realistically evaluate your own capacity and try to operate within it. Could you reduce your work load while studying?
Could you quit working altogether and finish school full time?
I'm not actually asking you these questions to answer, I'm posing them as the types of questions you can ask yourself to push yourself out of the parameters that you have likely set for yourself as givens in this current situation. Meanwhile almost nothing is ever actually a given.
Life is a series of trade offs and if your current trades aren't producing happiness, they might be bad deals for YOU.
Just because you *can* do everything, doesn't mean you should and doesn't mean it's the smart or right thing to do. A radical change in plans might be worth considering.
I guarantee you, life will never put external pressure on you to be happy and healthy, so the only pressure to defend your own well being will only ever come from yourself. So if you aren't putting pressure on yourself to protect your own health and happiness, no one will. The only way you can ever truly fail yourself is to not care enough about your own well being.
The most powerful question anyone has ever asked me is : "whose job do you think it is to make sure that your life is a good one?