Try to think of your disability as an excuse not to be so hard on yourself. You're not working because you need to focus on your health. It's not your fault, and previous employment would have just complicated your ability to get better.
As for your resume: you should absolutely put your volunteer work on the same level as paid work. Some potential employers might not feel the same way, but I guarantee you'll do better in the job market by presenting an image that you
believe the work was equivalent to paid work.
Feel better. *hugs*