Your niche finds you. Or it doesn't. Either way can work out fine. Sometimes your niche goes away (Flash programming for example). Often diverse experience is useful in solving novel problems.

You hone your skills by working hard for many years. You are just starting out. You have fifty or more years of career ahead of you. Keep in mind that people skills are more valuable than technical skills. People skills don't become obsolete. Other people are the best way to good jobs, interesting work, and good times. Be someone people want to work with again.

We are mortal. Nobody is invaluable. Many people are very good. Some are happy.

Good luck.