Welcome to the community, Tara!

People have written books' worth of material on finding one's place in effective altruism, but here are a few sources you might find helpful (if you haven't seen them already).

• 80,000 Hours gives EA career advice to people from many different backgrounds, including advice on which degrees to pursue. Their career quiz will give you a sample of options that might be a little better for you, though of course you'll need to modify their advice to fit your circumstances.
• Kelsey Piper, who writes articles on EA topics for Vox, has an excellent blog where she often discusses ways to balance health concerns and other realities of life with the desire to do good. Here's one post of hers along those lines.

Also, I should note that there are quite a few people within effective altruism who have serious health issues and have still been able to make valuable contributions. You're not alone!

Although not everyone will agree, I think focusing on things that are effective (likely to have a large positive impact on the world as you assess it), personally motivating, and fits well with your talent/comparative advantage is the best bet. Starting late seems not a major concern; if anything EA is sometimes hurt by the fact that so many people in EA are young and inexperienced and so waste time on things or make mistakes that a more wisened person would not. Having your life cut short is tragic, but losing a decade at the end of life seems not particularly relevant to EA; whatever good you can do in the time you have will help the world.

Given your health concerns, you might be particular interested in anti-aging research or cryonics, which I think of as effective good-doing since they have the ability in expectation to dramatically increase our capacity for positively-interpreted subjective experiences (although only in combination with other things). But I think whatever you chose to work on you have the potential to have a valuable impact.

I'm sorry to hear about your illness but it's good to hear about your interest in EA! I imagine it'll be hard to give concrete advice without knowing more about your background, interests, and skills. Do you mind giving a little more information?

You might find some of the general advice on the website of 80000 Hours interesting, although their advice is targeted at a slightly different demographic.

You may also want to join a local EA group in your city or university, if there exists one :)