Hey - I’m so sorry that you’re having issues with your wrist and tendonitis. I had and ignored this for YEARS until it grew so bad I got surgery. I only had surgery on my right wrist, but had issues with both hands. I managed to heal my left one on my own.
I am absolutely NOT a doctor and you should consult with a hand specialist. These are few things that helped, and continue to help, me:
Try to eliminate any movement that has your wrist bending forward (towards your inner forearm). That’s the position that tightens up the muscles around your carpal nerves.
Try wearing a wrist brace while you sleep. This will help keep the wrist from bending as described above. Many of us aggravate our wrists by putting them under our heads/pillows etc while we sleep, so this will help keep it neutral. Also, make sure the brace is one of the ones with metal ( ) support strips, not the cloth ones.
Take a look at what else you’re doing that might be aggravating your wrists. Keyboard/mouse hand positions are a big one. For me, it was keyboard/mouse and lifting weights. I felt better once I adjusted my approach to those things.
Do your shoulders/neck hurt as well? If so, it may be related as the nerves run all through there. Adjusting shoulder position/posture can help.
Strengthening the muscles in your forearms, shoulders, back will help too. Just make sure you’re not aggravating the pain by doing so. If you feel a sharp pain or tightness, then adjust your approach.
Adjust your position while playing guitar. Maybe your strap needs to be higher or, while sitting, the guitar needs to be in between your legs instead of across your thigh. Also check where your forearm is resting on the guitar, you might be repeatedly pressing into a nerve or limiting the support your forearm can give your picking.
For fretting, I changed the type of necks I play. Thicker and wider ‘C-shaped’ necks hurt my fretting hand less. Might want to experiment with different guitars and see if that works.
I hope you feel better soon!