If you have a very bony heel, as in the picture, you may have a Haglund’s deformity. This is where you grow extra bone in response to loading or just genetics.
These can be difficult to settle down because each step you take you are pushing bone into the tendon.
Please seek advice and do not keep pressing on with the exercises whilst there is pain as you are just continuing to damage the tendon. There is lots that can be done but it is much easier and quicker without a ruined tendon!