I understand entirely - but I think with INTPs that tends to click in later in life when you look back and start to connect the dots of what is actually important and what you are good at. We are too complex for easy answers to career etc. Hang in there and allow it to emerge rather than force it or despair!
you want to do something worthwhile - go help out in a homeless shelter or something similar. Learn a trade and go use it in a developing country. Find out which elderly people in your community need help with shopping or clearing snow from their path. Seriously - doing something 'worthwhile' is easy when there are so many needy people in the world. Don't let what you cannot do get in the way of what you can do.