How do i reverse the affects of forward head posture and upper back muscle tension on my breathing. i can't breathe through my diaphragm.

Physiotherapy: Perhaps the most efficient way to learn proper techniques to achieve appropriate posture and reverse any deformity would be a consultation with a physiotherapist. One can learn the exercises which have proven effective in correcting abnormalities and improving your breathing status.

PT: Physical therapy would be a good starting place. You might need cervical traction, or other modalities. You'll need a referral from your doc. Best of luck!