Since I was a teenager, I had feelings of despair and unworthiness and wanting something more. What shouldi do?

Psychotherapy Help. If you have been experiencing these issues since your teenage years, it would be very important to work with an experienced psychotherapist to discover the reasons as well as find possible solutions. Though there can be many possible causes, feelings like this can be common in people who have been abused or neglected as kids. Poor self-image and inability to move forward in life can result.
Reach out for help. It’s a long time for you having these feelings, but relief is possible for you. Talk to your doctor about these feelings, and talk to a psychotherapist. Stick with treatment, and if your feelings persist, share and discuss with your health care professionals so that treatment may be adjusted as needed. There is hope, just reach out for help.
Enough ! You deserve an evaluation by a well trained psychiatrist. Get a referral from your pcp. No more denying yourself.
Psychotherapy... These sound like symptoms of depression... Talk to a psychotherapist and explore these feelings and any underlying issues... Wishing you well...