Okay, so, this is something I have a LOT of experience with, I'll do my best to offer some advice:
1. Get it examined. Just because the pain is being felt in your wrist, doesnt mean the PROBLEM is in your wrist. "But I got it from too much controller use, so the damage/problem must be there", you might say, and to that I say AHA! That's what I thought too. But I soon learned otherwise, thanks to examinations by a doctor and my physical therapist. All the braces in the world arent going to do a bloody thing whatsoever if they arent being applied to the right spot. On that note, dont assume that it is tendonitis or any specific thing. Right now, it is an unknown, until you've had it looked at by a professional. Note that even too much controller use, if that is the original cause, might have caused damage of a type you arent expecting. It aint necessarily in your wrist, however it may feel. Seriously: Professional examination. I cant stress that enough.
2. Avoid too much usage until an examination has been performed. I mean, duh.
3. How's your chair? How's your posture? Believe it or not, these things can directly play into exactly what you're experiencing. Changing them up aint gonna magically solve it in an instant, BUT, if something is off, it could be a contributing factor. Armrests (or the lack of them, depending on what's going on) can be a problem in particular, but it depends on the situation. One way or another though, a good chair, and good posture, is going to go a LONG way towards helping you deal with this as time goes on.
4. Get one of these lovely things:
This is a vertical mouse. While you may think that the controller is the full cause of this, I guarantee you, your mouse is playing a part whenever you use your computer. Normal mice are not REALLY designed for full ergonomic use. Oh, they say they are. But they arent. They contort your wrist/arm/hand into a very unnatural position, and this builds up problems over time. A vertical mouse does not do this.
I'll put it this way: My problem is similar to yours, and I outright cannot use a normal mouse. But this? I can use this.
In all honesty, ALL computer mice should have this type of design. All of them. I'm baffled that they dont.
And yes, there's a bit of a learning curve at first when you get one, it's gonna feel weird and you'll have trouble pointing at things. But it doesnt take long to get the hang of it and it's worth the effort.
5. Okay look I know I mention the water thing a lot, but I'm mentioning it again here, because if you've gotten at all dehydrated, it's only going to exacerbate this.
6. Use ice! This is to keep inflammation down. Just, you know, do it in a way that doesnt put pressure on it, and in a way where you can hold your arm in a neutral position the whole time.
7. Get up! No, seriously, just get out of your chair and walk or pace or whatever every now and then. Every 40 minutes, is what I was told. This is just overall healthy, and "overall healthy" is gonna do nothing but help you here.
EDIT: quick thing: Even if you're using a laptop, the mouse bit is still important. I guarantee you, those bloody track pads or whatever are NOT helping here.
