What could cause sternum pain that lasts for three months?

Hello doctor,

I have been having pain in my sternum chest bone for approximately the last three months. Then I went to an MD medicine physician, he got my X-ray done and it was normal. Then I got my ECG done, and there were no palpitations or spikes. It was also normal. The last test he did was an echo the report was normal. He gave me anti-anxiety pills for 15 days and an Ultracet tablet for pain relief and suggested ice-pack therapy thrice a day. Still, the pain is the same, it worsens when I squeeze or directly get up straight from bed without turning left or right. For a second opinion, I also went to an orthopedist to clear out if there was a fracture or a little bit of crack, he again did an AP-lateral chest x-ray which also came out normal. No costochondritis or fracture is there as per the doctor. At last, I got my CBC and c-reactive protein test for infection and LFT, which are also not elevated. I got all possible tests done and got my checkup done by a cardiologist, orthopedist, and MD physician, but not able to identify the cause of the sternum pain. It is very sharp and throbbing so cannot ignore it. Is there any doctor who can help me out in identifying the right cause?