1. I'm 20 years old.
2. I'm a music student, but currently have no job.
3. I broke my right wrist when I was 16, but it doesn't affect my playing so far as I know. The only thing I can't do with it is hold things flat (boxes, plates etc.)
However, I have got a bad back and neck (I have a spasm in my Trapezius, and general neck/back strain/discomfort/pain if I have to sit upright or stand for too long, 15 mins or more I suppose. Pretty bad for my age!) I play a Stingray too, and the weight of that is killing my back and shoulders at the moment.
4. I think in order from most to least it would be: Computer keyboard/mouse > bass > video games > carrying stuff? I'm not sure what else would be relevant.
Great stuff and a question straight back,
have you seeked out medical treatment for the back and shoulder problems? IF not why..why do you tolerate this problem?
You have pretty much a young persons modern hand usage and the problems that it brings, the neck problem is part of it.
Compare me to you at 20. Back in the 70s playing bass was the most hand usage i had, so it developed pretty much un-involved with other things. No video games, mobile phones, no push pads or touch pads of any type, no computer keyboards, no mouse, no remote controls...so pretty much it was just bass for an hr. or so a day.
You probably have more hand use in a day than i would have had in a week, and not good hand use at that.
Take out the bass and your hand use is one way, using one action in your fingers. Because this action needs you to visualize you neck and shoulders are always positioning the head to cope with this so your neck and shoulders are what we say in the "up position" a position many will carry and hold after they finish the task. You have to be aware of keeping everything down, that is relaxed. Find a mirror and look at yourself and make you shoulders go down, relax them, and your breathing and take the tension out. Learn to keep them down, it takes practise, the first step is to be aware that it can happen, then you can correct it when it does.
For you the hand stretches of opening your hands wide and holding it, then relaxing, and the hands flat and lift the fingers up alternately are what you need. You need to un-do your day before you play, like the sprinter in the analogy you are already tired before you try and play/practice. You need a warm up to help you do this.
Below are some links for you to watch, see if you see your problems in there and take the relevant exercises to form part of a warm up you repeat every day and before every time you play or work and when you finish. It should be a short
un-intrusive routine easy to repeat and addressing the points you have. Just on what you have told me i see your problems not as needing more exercise, but the opposite, you need to stretch more to take the tension out of you muscles in order to make them soft and supple and able to work to their best.
Never suffer pain, it is there because something is wrong, see a doctor or physio about such things, because when you get older you will soon realise that you should have acted sooner.
But that is what being young is about shrugging of things and getting on with it so you can tell to other young people when your older about how you now realise your mistakes and never listened to those who warned you.....it is an ongoing cycle as i am proving here.

Any question drop me a PM or post, happy to continue to help you understand your situation.
Here are some links,
neck stretches