From what I have read, most people define themselves by their job. To my mind this is no better than defining yourself my your brain tumor. In a less than perfect world, I don't see a problem either way. However, since you have decided that this is not what you want, I commend you.
So, how will you define yourself?
However much time you have left in life, it is yours to define. Through love, action, the sharing of your life and your story, you can take your tumor from being your identity to being a small part of your identity. A small, but important part of the story.
Try to be positive and embrace all the good things in life, focusing most on the hundreds or thousands of things you can do, not the few things you can't. We are only limited by our willingness to accept limits and do the work of healing that is required.
Following Suu's advice is also a very good thing to do.
Tod