Advice for requesting a reference for summer nursing internships?

I'm applying to summer student nurse internships. One of my references needs to be from my current clinical instructor. I think she likes me alright and I have no choice but to use her anyway. A second reference is also required, and I can choose who writes it. I worked in a student job for 2 years and always had a good relationship with my boss but I accidentally missed a day my last week d/t mixing up of schedules. I also haven't visited work like I said I would do, although in my defense I've only been in the vicinity twice and only for one day when the office was open each time, and I was just too tired from travel to go visit.

I haven't been in touch with my boss as much as I'd wanted to be after I left (we were friendly)... now I feel like it's awkward to ask for a reference out of the blue. He's a great writer so I'd love to use him, but don't want to seem selfish/disingenuous. Alternatively, I could use a different previous boss who was left with a better impression of me, knows me less well and is a worse writer.

Thoughts? Ehh... reference-getting is so stressful.