I did like a lot of the things about my mission--getting to know a new culture and gaining a better awareness of the world. Learning a second language was a good experience (though admittedly I haven't used it at all since being back, though it was a good factor in getting hired in one of my past jobs). Getting outside the Utah bubble and talking to people in the world really opened my eyes.
Some missionaries go into their mission thinking they will be protected in every way if they are righteous. This can be a harmful idea. You still need to take care of yourself! Pay attention to what kind of traveling you will do in your mission, in my mission it was walking and public transportation only. I still have issues to this day (got back from my mission about 8 years ago) where my feet are really achy sore in the morning and I hobble around a bit, and I wish I had better shoes for my mission. Some missionaries come back with more serious lingering health issues (diseases, injury, etc) than mine.
Also some people talk about it like they were transformed into this new confident person on their mission. For me, I still have anxiety issues that stem from mission experiences. As if constantly getting rejected when going door to door and person to person isn't enough, I went to a part of the world where there is a lot of anti-American sentiment and had a few scary experiences with that. Many missionaries struggle with things like homesickness, faith crisis, the strict 2-year lifestyle, etc that will force them to re-evaluate what they are doing on a mission. There are lots of possible sources of stress when you're out there, it hits some people differently than others.
Also, don't expect anyone to be perfect on your mission. I had a MTC teacher who used really manipulative and mean tactics to try to get us to work harder. You may have some companions that are challenging to get along with too.
Not trying to scare you but I wish someone had given me a more realistic idea of what to expect before going on my mission. I've heard that statistically at least 1 in 10 missionaries don't make it through their entire mission, and I have several close family members that have come home early for various reasons. So you can expect good things but you can also expect to struggle in more way than one.
My recommendation is that you don't go into it with expectations that it will make you a better student, or have better success in your future career, or have better relationships with people, or anything else. Your mission experience will be unique and you will have your own unique consequential benefits as well as problems depending on many different factors, some of which will be in your control and some not so much.
Best wishes!