Not because I think I have all the answers but because simply going through it is a whole-life-encompassing process, I feel like I have learned some things that would have been better for me had I just steered clear of it.
1. Comparing your “application” to others’ applications- by application I mean you as an entire package. Your personality, grades, extracurriculars, work experience, etc. There is more than one way into medical school and you don’t have to fit the proverbial bill to get in. Say somebody has an amazing internship opportunity that you know. Just because you don’t have that, doesn’t mean you don’t have what it takes. Find your path into medical school. Chances are, you’ll have something in your life that no one else will have.
2.Not resting- You need to take a break. Even if it means you say no to a study group. Resting can mean sleeping, playing frisbee, or simply passively studying without freaking out. It’s easy to say now that I’m in but I look back and see that I really could have thrown the book on the floor and gone and played raquetball a little more or fully focused on having family time instead of trying to study and missing out.
3. Having a job-I guess this is a personal preference/need but I tried working a part time job during undergrad a couple times and just never could make it work. I found myself going without brushing my hair and eating regular meals just so I could “maximize my time”. Bad idea. I just ended up gross, tired, and hungry, which isn’t a good state in which to study or live for that matter. So I applaud you if you can have a job and make it work, but I would advise against it.
D.O. on the go 
My hope is that I can advise to one day have a job. In my opinion, it’s a good idea.
Comparison steals your joy!