I irritated/ inflamed my urethra through vigorous masturbation. my doctor advised it will heal over time with rest. i wanted to ask is this true or have i permanently damaged my urethra. feeling terrified?

Hmmm... this is peculiar. I've cared for thousands of cases of urethritis and never heard of it being caused by masturbation (not counting cases where objects were inserted into the urethra). If you have sex other than masturbation (i.e. with sex partners), I hope you were tested for STIs; and that antibiotic treatment was given. But regardless of cause, long term damage is very unlikely.

You have not done any permanent damage, Try avoiding touching the underside of the penis and use a lubricant if and when you matyrbate. You will be fine.