I have constant pain on the underneath of my heels to the point where i can't walk on them?

Plantar Fasciatis: You may have plantar fasciatis. See your podiatrist. I am sure you will be helped.

Plantar fasciitis: Plantar fasciitis (heel spurs) or fat pad syndrome caused by overuse of the plantar fascia or arch tendon of the foot. Usually worse in the morning or after sitting. Tennis elbow of the foot. Stretching, antinflammatories, steroid injections heel cups, different shoes and occasionally surgery may be helpful. Takes a long time to treat successfully.

Likely plantar: Fascitis. Try icing the heels, stretching the achilles tendon, wearing arch supports, taking anti-inflammatories, massaging the heel before standing after periods of rest. If pain persits see a podiatrist.