I had a 5 year relationship from about 17-22. A lot of it was long distance. She broke up with me and it felt like my world ended. So here's my advice as somebody who was in a similar situation.
Hey, guys, I'm a 21 year old man, who has been engaged to the same girl since 2016.
Just my opinion, but 19 is probably too young to be getting engaged. I am 31 now and I was literally a completely different person when I was 19. Most people have absolutely no idea what they want at that age, and committing to a single person before you really know what you want in life is probably a little ambitious. So try to look at it from that perspective. It's a blessing in disguise.
She left our hometown and moved 1300 miles away to go to school in Scottsdale, AZ at MMI. We've been long distance for a huge part of our relationship, and we made it work.
Long distance can work, but rarely does. As I mentioned above, I had a long distance thing with my high school sweetheart that worked for like 3 years, but it eventually ended. The distance takes its toll and people drift apart.
She just got back yesterday, wasn't even in town for 24 hours, broke up with me, packed up her shit, and left me in an almost empty apt. What do I do from here guys?
Relationship wise? You grieve for a while, maybe months, but then it gets better. Then you move on and start dating other girls.
I have one semester under my belt in school, and haven't been in nearly 3 years. I'm fat and out of shape.
There's no real easy way to say this but... you don't have a higher education and you are admittedly out of shape, so are you really surprised this girl called it off? What were you offering her from a long term perspective? How were you planning on providing for your family from both a financial and physical health standpoint?
Your next step should probably be to try to get your degree, however you can. Focus on that first and foremost. Then join /r/fitness and look up the introductory posts. Start working out. Then start paying attention to your diet.
I don't even remember how to date.
Neither did I. There's unfortunately no shortcut to learning, so just go do it. Lucky for you, there are dating apps now. Sign up to them and start chatting to some girls. Just don't come off as desperate -- that was a big issue for me when I was on the rebound after my relationship ended. I just wanted to find "the one" immediately and gain back that level of intimacy I had with my ex. It doesn't work that way -- just get to know people as best you can and let stuff happen naturally.
I don't remember how to deal with this kind of grief.
It sucks. There's no way around it. There will be good days and bad days, and you just gotta survive them. Working on improving yourself helps, and keeps you distracted. So do the education/fitness things above.