"Dear Natalie,
I think I've bitten off more than I can chew this summer. I'm taking two summer courses, volunteering, and I'm working full time. I don't know what to do because I want to spend time with friends and family but at the same I need the money to do those things."
I feel you. As someone who's probably also bitten off more than she can chew, I really, really feel you. But I know only my own intentions in a situation where intentions are kind of important: I need to work full time to pay rent and pay for school, so when I graduate I'm not drowning in student debt. I probably still will be anyway, but I'll do my best not to let that happen.
But every case is different. What could be just enough work for one person could be far too much for the next. Your case though, gosh, it does looks like you do have a whole bunch on your plate and honestly it may be too much.
Ask yourself some questions. Do you need the work? Do you need to work full time? Are you stressing over your finances before every pay cheque? If yes, you will have to, in the words of my mother, "just stuck it up, it's only for the summer."
Your friends will understand if you can't make every event, because chances are they're also working, and can't either. Set up a Doodle so you all can find a time to see each other. Same goes for your family. They should understand that summer is for working -- even though you're taking classes too.
But if you don't need the money and if you really don't want to spend your summer indoors, then ask your manager if you could get reduced hours (cite your course load as the reason, rather than your failing social life. It might go over a little better with the big boss).
But if you do this, and you do get reduced hours if you don't really need them, then no complaining in September when you have less money to spend on frat party tickets.