I have a long and sordid history with plantar fasciitis and tendenitis in both feet. I have had cortisone shots in the heels, the post-tibial tendons, the side of one foot and the tops of both feet. Sometimes they work and sometimes they don't. I have done the stretching, which does seem to help but it's pretty painful until you get the heel cord more flexible. I have been in a cast, on crutches, and in a walking boot, just trying to stablize my stupid feet. I have been on anti-inflammatories daily for many years. At this point, I have severe arthritis in both feet and it will not ever be any better than it is right now. Which is not good, let me tell you.
The one thing that really has helped is good, custom orthotics. They are not inexpensive. In fact, mine cost upwards of $300. But I wear them every. single. day. I no longer wear pretty shoes or sandals because my feet quickly fatigue and I can hardly even walk. I wear New Balance exclusively, to church, to weddings, to the beach, even to fancy dinners. If I don't, my feet will give out within 20 min and I have to be wheeled to the car.

I had to buy the NB shoes at a special NB store because the standard ones you find in retail stores simply don't have enough support. At the NB store, they are fully educated on fitting shoes specific to your needs. There are 2 shoes there that work on my feet. I buy one pair every year on my birthday--they are about $140. I wear them until they're worn out.
Good luck. It took my about 4 years, initially, to get control of the pain. I have to stay on top of it all the time or the PF and tenditis return with a vengence. If you can possibly afford the orthotics, try them. It's not guaranteed to work, but if nothing else is working I'd give them a shot. It takes anywhere from 2 weeks to 2 months to adjust to them, so be patient with the process.