I've got both plantar fasciitis and Achilles tendinitis. I've been to the gp and I've been to the podiatrist twice now. My arches fall down and i was given insoles which I wear all the time. I do all my stretches but I am still in horrible pain! The podiatrist has now told me to cut down on stretches because She thinks I'm damaging things more so I'm only allowed to do little tiny stretches. I'm always in some degree of pain and I can't go on any walks longer than ten minutes. By the eveningI am unable to bear weight on my bad foot and it no longer stops hurting when I rest my foot. The Achilles' tendon keep on "twanging" which feels horrible and aggravates the pain. I've read about the possibility of surgery and was w done ring if anyone has experience of it. I'm going back to my go soon but they tend to be a bit dismissive. I have fibromyalgia as well and they tend to blame everything on that.
When I had plantar fasciitis I saw a physiotherapist who taped the sole of my foot (a really wide non stretchy support tape) underneath the arch. I was sceptical but it really took the edge off the aching pain. She also got me a small spiky ball for rolling under my feet and along the outside of my calves to relieve some of the tension caused. I don't know anything about achilles tendonitis though or how to treat. Do you need to treat both together? Good luck!
Also, are you on any medication like Levaxin, for the thyroid, I reckon from experience there's a connection between plantar fasciitis and the Levaxin treatment, and when I stopped using Levaxin/Levothyroxin (I had it only briefly) the problem with plantar fasciitis disappeared completely, but that's just in my experience. I figured it had to do with the level of the thyroid hormones, but that's just my guess, my opinion although Google tells me I'm not alone in making the connection.