It takes a long time for a tendon to heal. (no pun intended)

Northcoast Footcare
This link is one of the best online. It has many categories of foot problem, but the heel tendons is one of the first options on
the site. It also has a good anatomy page too, which has information about the location of the nerves, and ligaments.
This comes in handy for using topical agents.

Have you soaked in epsom salts? This is very helpful in providing needed magnesium to the feet.

Have you taken fluoroquinolone antibiotics? Cipro, Levaquin and Avelox disrupt the maintenance of tendon metabolism and months or years later can result in a tendon failure/rupture.

This family of drugs is very damaging to nerves too, and the jury is out whether they cause permanent damage or may be reversed over time.

You can also try Salonpas patches (the small original ones).
You can stack two on your affected tendon at night and take them off in the daytime. I had a tendon problem many years ago and found these very helpful after using 7 days in a row.

You can also for pain try the new Aspercreme Lidocaine lotion.
Rub a small amount in once a day, say in the morning opposite the Salonpas patches and see if things improve some. Aspercreme also makes a lidocaine patch now... so make sure you get the Salonpas with the methylsalicylate in them. These have anti-inflammatory actions which lidocaine does not.

This is the Salonpas patch --they make several products and this is the one I recommend.
Salonpas(R) Pain Relieving Patch | Salonpas(R) Pain Relief Products

This is the link to the Aspercreme Lidocaine lotion:
Aspercreme Pain Relieving Cremes and Gels: Pain Relieving Creme with Lidocaine

This Lidocaine is better IMO than the new OTC patch because
the unusual location would not lend itself to the Lidocaine patches, which have an odd adhesive which is aqueous based and doesn't stick well for many people. The lotion when rubbed in well works much better.

The Salonpas have anti-inflammatory action and can be therapeutic, for healing. The Lidocaine is only a symptom reliever but very helpful for relief. I use both when my feet flare up and have had good effects.

The magnesium in the epsom salts opens the circulation to the feet and promotes healing and also blocks the NMDA pain receptors.

If you respond to these simple topical agents, you may recover depending on the cause of the pain in the first place.