How can I regain my motivation and my drive?

You already quit before you started. You seriously think motivation pushes people through life? Do you really think that successful people wake up every day motivated as hell? Every. Single. Day?
Motivation is a lie. You didn't go to basic to learn motivation, you went to learn discipline. Identifying goals and setting conditions for reaching those goals. Determining a course of action to reach those goals and the DISCIPLINE to move forward in those steps day after day until you reach the goal.

You are in literally the best MOS for a junior Soldier. It's the most fun, you get to do the fun stuff with your guys, you get time for college, and it opens a door into many other areas. Best of all, you'll never experience any bond like the bond between a platoon and their Doc.

If you don't like where you're at, then change it instead of feeling sorry for yourself. It doesn't matter where you're at, if you're a medic there are opportunities for you if you're willing to put in the work. And if you REALLY want Air Assault then go to Fort Campbell, it's practically part of inprocessing there.

Everything unattainable? Why?

So, I'm just going to be blunt because this generation of young Soldiers need to hear it even if it offends them or hurts their feelings. It's called being a Soldier and a professional. That's how you find the motivation. So, the Army is not exactly what you thought it would be...welcome to the real world! You signed a contract so man up and honor that contract. If you can't do that, then it's safe to say, you're going to do the same thing in life when things get hard. What are you going to do when marriage isn't what you expected? Or when being parent isn't what you expected?