What you're feeling is normal, and will pass with time. Thinking about one's purpose in life can be stressful in itself, and I know that because I've been there myself. It just seems like I'm not as productive, not as motivated, not as GOOD as the person next to me, but that's just false! We all have our own ways of motivating oneself, because we all have our own desires and wants. As long as you keep chasing what YOU want, you'll figure out a way to notice how useful you are. No matter what happens, you are an important individual, and whatever you do, you are doing the best you can. It's not as hard as you think to start volunteering. Volunteering is a great way to get yourself involved in the community, and to see the world in a new perspective. You will see how important of a role you play.
Start to love yourself and embrace your strengths. There are people who are ready to judge others. So its not necessary to just go in the flow like others , all are different and that makes all unique. You are different from others and have strengths also. What others can do, may be you can't , yeg whag you can do, may be they can't. Hence love yourself, you are not useless, you have good capabilities, just use them to make your life good and blissful.never underestimate yourself. Just love yourself, discover your hobbies and be cheerful all through your life.