Advice for quitting my job - feel so guilty!

I have been in my current job for three years and although I enjoy it (most of the time!) and the people I work with are lovely, I have recently been offered a fantastic opportunity to do something that I've always wanted to do, and will help me to pursue a career doing what I love. The only issue is, I'll be leaving my job at a really busy/difficult time. I'm in a small team and another colleague is also leaving so me leaving as well will really screw them over! I did mention the new opportunity to my manager a while back, but at that point I stated that I didn't think I'd be successful in getting it (and I also think he's forgotten all about it!) He knows about my other colleague leaving of course and keeps saying things like "at least you're staying" etc. and I honestly feel so awful! Recruitment is really slow at my workplace so it'd probably be a while before they could hire new people. How do I go about handing in my notice?

I have been in my current job for three years and although I enjoy it (most of the time!) and the people I work with are lovely, I have recently been offered a fantastic opportunity to do something that I've always wanted to do, and will help me to pursue a career doing what I love. The only issue is, I'll be leaving my job at a really busy/difficult time. I'm in a small team and another colleague is also leaving so me leaving as well will really screw them over! I did mention the new opportunity to my manager a while back, but at that point I stated that I didn't think I'd be successful in getting it (and I also think he's forgotten all about it!) He knows about my other colleague leaving of course and keeps saying things like "at least you're staying" etc. and I honestly feel so awful! Recruitment is really slow at my workplace so it'd probably be a while before they could hire new people. How do I go about handing in my notice?