Are you crazy? Yes, you are, so don't worry about that.
But, going to medical school as a non-traditional is an experience that I wouldn't have missed for the world, so - welcome aboard.
As a 47-year-old MS-IV, what I'm thinking is - there is no way to explain to you what's coming. All I can say is, you've made it this far in the process so there's about a 99% chance that you'll make it through just fine. You already have the "right stuff."
My own experience was - I thought that, after two years of pre-med courses at night while working a full-time professional job during the day, nothing could phase me. I was wrong. The memorization skills required in the first year of medical school were intimidating and overwhelming. My own adjustment to medical school was quite difficult - I lost of a lot of weight first semester and cried a lot - and wondered what in hell I'd done to my life. But, I adjusted, I rose to the occasion, did well, and survived. And you will, too, or you probably wouldn't have gotten in as a non-trad. By the second semester of first year I was pretty much an old pro at doing what was expected. Second year was much more interesting and more fun and I knew what to expect. Third year - well, as a non-trad, I was perhaps even more exhausted and stressed than some of my classmates. But I was finally beginning to practice real medicine on real patients - and I loved it. And you will, too!
Try to be flexible. Your lifestyle will change more than you can imagine. It's tough to adjust to having very little social life... and spending most of your days studying. You will recognize almost immediately that the change in economic circumstances is no big deal, because you will no longer have time to do things that cost money (I'm not trying to be funny - I'm serious about that). I used to care a fair amount about my hair and my clothes but in medical school that quickly became dead-last on my list of priorities - I don't know how many mornings I went to class with torn jeans with dirty hair under a ballcap and couldn't have cared less.
Lastly, TALK about your stress. Your classmates can be a huge source of emotional support, because they're going through the same thing. Your sig other and family will be a great source of comfort, but you'll get frustrated because they'll say things like "it can't possibly be that bad" when it bloody well is. One of my teachers said in my first year "only a physician or a medical student can understand what it's like to go through medical school." That's very true, so don't waste time being frustrated that your family can't truly understand how you feel. That's just how it is.
Enjoy your summer. Welcome to medical school. First year is going to be the rollercoaster ride from hell - but there are plenty of highs to go with the lows.