Possible reason why the arch and inside of my foot hurt so bad?

Soft tissue injury: Common pathologies for arch pain, particularly on the inside of the foot are plantar fasciitis and posterior tibial tendonitis. See a podiatrist to have an examination and have options for treatments.

Ligament/tendon: You may have plantar fasciitis which is inflammation of the plantar fascia ligament which is a broad ligament in the arch. You also might have posterior tibial tendinitis or dysfunction. You need to be evaluated by your pcp, podiatrist or orthopedist to know for certain. Once this is known, then proper treatment can be rendered.