I have tendonitis and tenovenitis in my left hand and thumb due to a work injury. It's been that way since 2003. I've had surgery to release the band that surrounds the tendons, 5 cortisone shots in my wrist and 3 in my thumb joint. I know exactly how bad you have to be hurting to get those shots. Here's what helps for me.
Polar Ice is what I think it's called. It's a gel that doesn't smell too good, but it like freezes the area. You can buy it at the drug store.
Go to a Tractor Supply store or anyplace that sells Vet Wrap. It's what trainers use to put around the horses legs. This will give you support without causing you to lose your mobility using the splint. I can't do ANYTHING wearing the splint. I have a molded plastic one, a Sammons glove (blue in color), a velcro black one that has a metal splint in the thumb area. I think my hand hurts worse when I wear them.
I also invested in a Hand and Foot Spa. You melt parafin beads in it and then dip your hands in the wax. I usually dip about 5-6 times and then let it cool and peel off. It brings relief especially after using my hand alot like when I cut out my quilts.
I put my binding on using the sewing machine, but then hand sew the other side on. After I do this my hand hurts for at least a week. It usually takes me about a week to do a queen size quilt.
I also use a rice bag. If you don't have one, it's easy to make. Cut a piece of fabric, fill about 2/3 full of long grain rice. Sew shut. Heat in microwave for NO LONGER than 2 minutes.
I hope one of these things help you. I'm right there with you. Cheryl in Ohio