Can Diastasis Recti separate all the way to my sternum? Can it cause it to bulge with severe pain?

I am sorry to hear about the problem you are experiencing. Yes, diastase recti  can extend from the sternum all the way down to the pubic bone. This entire length of the abdominal wall is addressed during tummy tuck surgery (plication).  Whether or not the presence of the diastasis is causing your pain should be evaluated by your physicians in person. Careful  history, physical examination, laboratory study, and imaging may be necessary to do so. Best wishes.

The rectus muscle attaches to the ribs and the separation can occur that high. A careful examination can verify this. A negative CT scan helps rule out a true hernia, which could be repaired during an abdominoplasty.

Diastasis can extend all the way up as you describe.  The CT seems to confirm this.  Repair of diastasis is usually performed with a tummy tuck. In sever cases, mesh may be indicated as well.Consultation with a plastic surgeon is recommended who may also involve a general surgeon.