Hi Jacob,
I think you might really enjoy and benefit from reading this blog by Julia Wise. While it's great that you have such a strong instinct to help people, we're in this game for the long haul, and you won't have a big impact by feeling terrible about yourself and feeling guilty if you don't make sacrifices.
In particular, it's very likely that focusing on doing well in college and then university is going to make a much bigger different to your lifetime impact than whether you can get a part-time job to donate right now.
Hi Jacob, this kind of stress is very relatable for a lot of us. Many of us handle it by thinking about our time and finances once a year, deciding how much is for effective charity and how much is for us, and then sticking with that for the rest of the year.
For example, I donate 10% of the money I make to charity, and I use weekends and evenings to relax. Some people have a different standard - for example, donating everything above what they need for a basic life and only taking one day a week fully for themselves. Ultimately it's up to you, but setting some limits for yourself can hopefully make these decisions less stressful for you!
One further thought: you're very young. It would be great if you could get a part time job and donate £1000 this year - you could make a huge difference for someone! - but it's probably better to spend this year setting yourself up for a good career later on. Doing well on your A-levels and getting into a good university or trade program, so that you can earn a good salary when you're in your 30s and 40s, will probably allow you to help others more overall than working part-time now.
I also thought you might be interested in this blog series which a lot of us have found useful, called Replacing Guilt: http://mindingourway.com/guilt/