Stop doing what is causing you pain until you fix your posture/ergonomic condiotions and try not to use your other hand to do the same stuff that caused you pain or you will end up with two aching hands/wrists instead of one.

My first advice for you is to get a mouse that fits the shape of your hand allowing you not to assume an unnatural position while using the device. Secondly you should also look for an ergonomic keyboard!

What you can do to ease the pain is using a bean bag, taping also helps but isn't necessary, most important is to take regular breaks in between your sessions. If you're sitting there for hours which isn't even extreme cases anymore these days you will want to do regular hand, wrist and finger exercises. There are many instructional videos on that on the major video-platforms and best if you find something that's going to work for you. Using a Gyrotwister or Powerball may also help improve your condition.

My best advice would be to consult a doctor when the pain doesn't go away after some time. They will find the cause and give sensible advice what you can do to improve your condition. :-)

Definitely proper positioning and regular breaks. I've had this several times, worst bout was when I was still playing DAOC (MMO). I did become the third max level EU Necromancer but it hurt like hell for days and needed a lot of rest before I could do long bouts again.

As for controllers, at least for me, once it starts getting bad, it really doesn't matter what you do anymore, any repetitive finger movement becomes painful eventually, including controllers.

Thank you (and the all the other users!) for all the information you put here.
I definitely going to check some of those advices (apart from consult a doctor that it was something I was going to do anyways)

I've been suffering from wrist and back pain for sometime now, but lately it got a lot worst and now I can't work normally without my right arm and backside hurting after just few minutes of doing any work/ gaming.

Added topic to favorites!

Ha, that's nothing! My uncle is dead, D-E-D dead!

Try to fix that with a mere wrist support or extra vitamin D. My uncle would LOVE to switch places with you!

Well at least his out of his misery!

Whenever my RSI gets worse I use Work by Speech for a couple of days to let my wrists rest.
This app allows using a computer by speaking to it only. You can read more about it here:
workbyspeech.com

WEll, these days you see even people defeat Milena ( everybody knows who Milena is right?? ) with a hola hoop. It does seem an easier solution to switch between controller games and mouse and keyboard games on a regular base.
Exercising does a lot too, especially exercises aimed at improving the strength in your forearm

There are a couple of things you can try.

Try changing from right hand to left hand (or the other way around).

Consider stopping with gaming entirely or on a different platform so you can play with a controller.

Maybe try a different mouse, a better quality one that better registers movement.

Or just put your jaws together and take the pain.

Amputate your sore limb, when it isn't there it can't cause you pain, on the other hand maybe not so good advice.

Last thing I would do is consult a specialist, usually they charge you a fortune and still solve nothing.

I was a keyboard jockey long before mice became a thing (I'm old). Soon after mice came out for windows 3, I started developing wrist aches. For me, the solution was the Kensington Expert mouse (a trackball with a large ball). Decades later I developed a bad habit of resting my ring finger on the scroll ring, which was most annoying. So I gave that up and started using a thumb ball. It took a while to become accustomed to it, but it works well.

If you go the trackball or thumb-ball route, look at the software which comes with mouse and adjust the acceleration so you can get from side to side of your monitor with a quick movement of your fingers or thumb. Microsoft's acceleration control gadget is barely useful, but it can be made to work.

It will feel strange at first, you might find old habits kicking in. Keep at it, you''ll become accustomed to it. As many others have said, carpel tunnel syndrome and other RSI maladies are not insignificant so take of this right away.