Disclaimer: I am not trying to pull the "I'm older and smarter" card, because to be completely honest with you, I still have no idea what I'm doing. But what I can tell you is that the person I was at 21 years old is 150% different from the person I am now. And it's only been 4 years! I thought I knew literally everything then, ironically. I've wised up a bit since. I'll give you a little bit of personal background information to highlight my own experiences, just in case you care, haha.
When I was 21, I was just about to graduate from college and move back to Dallas to start a full time job. At the time, I had a VERY serious long distance relationship with a guy I met in high school. We dated for 7 years, and I don't mean on and off either. It was a solid 7 years, and I thought I was going to marry the guy. And to clarify, when I say long distance, I mean we were in different cities. Not far at all, honestly. Only 2.5 hours away.
I moved home after college and he was still out at school getting his Master's. Nothing about our relationship really changed, but we started growing apart. And not for any real reason rather than a) we rarely saw each other and it became an out of sight, out of mind scenario, and b) the distance caused unnecessary drama and created issues that didn't actually exist. Slowly but surely, the relationship fell apart, as expected. And while that's all fine and good, I can also tell you that those years were the absolute worst and most stressful times in my life, and I wouldn't go back and suffer through them for literally anything. It was a loooonnnng slow death, and it took a huge toll on me.
Now, if we consider the other aspects of my life, they were all little-to-no stress, whatsoever. I had a nice 1-bedroom apartment in a big city, with a great first job to start off my career. I had a great friend group and a supportive family behind me. I wasn't hurting for money. The only real source of pain and stress in my life was that relationship.
Now, think about the mental stressors of OCS and the military in general. The physical stressors/exhaustion. The time away, no contact, etc. The studying, focus, intensity you're going to have to put into this whole thing. You won't always have time for this guy, unfortunately. And vice versa, as you mentioned he's military as well. There will be new uncertainties creeping in regarding you (a pretty female) in a male-dominated industry. And maybe he trusts you 100%, if so, that's great. But those thoughts will inevitably cross his mind at some point. That's just the way it is.
All that being said, I personally will not to go into OCS/the military while in a relationship, based largely on the fact that this journey we're about to embark on will be stressful enough as it is for us alone. Top OCS stress with relationship issues, and honestly, I'm not sure I'd make it out with my sanity. I personally don't want or need any outside distractions or emotional turmoil. I personally don't want to be training and have some guy cross my mind all of the sudden and pull me away from being present at OCS. It's too big a risk. Matter of fact, I've got to break things off with the dude I'm currently seeing...if I ever get my orders.
Anyway, just my perspective on the whole thing. Obviously, some people do have successful relationships in the military. It's not completely unheard of. But as someone who spent a large part of my life with a dude that's nothing but a memory now, I'd hate to see you make a choice that holds you back or creates added stressors in your life. I was in a relationship all through college, and as a result, missed out on a lot of fun opportunities and great friendships I could've made. I can't relive or redo that time in my life, and it's pretty disappointing looking back. You honestly will probably look back at yourself in 5 or so years and be amazed at how much things change. How much you will change.
Anyway, I'll get off my soapbox. That's just my two cents. But I'll urge you to really really really consider all aspects of the lifestyle changes you're about to be forced to make. Good luck!