Take everything you're about to read with a massive grain of salt. I am not a doctor, and you should seek new professional medical advice.
> But after playing piano for 20 mins
This sticks out to me like a sore thumb. Elbow tendonitis is super common for piano players.
> My doctor has no idea whats wrong with me, and said the body is a mystery.
Get a new doctor. That is, put nicely, the fucking dumbest thing I've read all month. That's absurd. You're describing tendonitis. Specifically, look up tennis / golfer / mouse elbow (the only difference is which side of the elbow is affected). It's extremely common, but also takes a long to time heal.
>I got the injury after playing video games and basically spamming the controller and typing.
So basically everything below your elbow connects at the elbow. See reference one and two. Typing, writing, grasping - all of that happens with your elbow at some point. The type of injury that you're describing though, that sounds like you're 'rotating' your elbow one way or the other too much. This is likely due a change in ergonomics.
An anecdote: In my case I had gained a fair amount of weight, and my stomach essentially started pushing my elbows outward. Nothing changed in my behaviour, but all of a sudden I had debilitating pain in my elbow, forearms, and wrists. It was very confusing, and took me a long time to identify what had changed - my gut.
I'm not saying that's your issue, but I am saying that this can be really difficult to determine how to treat. For you it could be as simple as you're body has matured into a new shape. You will still continue to grow up until about 25 years old.
>Should I just accept that I’m never going to get better? The pain is not excruciating, but it’s bad enough to break my concentration and make me stop whatever I’m doing.
Seriously though, don't trust anything on here you're reading, but do speak to a new doctor. You likely need time, rest, physio, and an ergonomic assessment. You can recover from this, but continuing to aggravate your injury will have long term detrimental effects and will prolong your recovery.
To help recover, you may need to use your hands and arms as little as possible. Look up Dragon Speech - it's a dictation software. It will help lighten your load, and works excellently.
I use this elbow strap (Hyperblend Elbow Sleeve with/Strap - you can get discounts if you buy direct from their store, but their main site sucks my taint). It's good quality. It dramatically reduced my pain, but long term it's not something you want to rely on.
Anyway, there's no quick fix to this. Unfortunately the only long term solution is rest, stretching, and exercise (you'll want to talk to a professional to avoid further injury).
Good luck!