Your feelings are valid because they're yours, but PLEASE try to not let that regret eat at you, ever.
You made the right decision for you ATT, and that's ok. Maybe that decision saved you to do smth else.

Re the justice of various wars:
THAT'S THE HARD PART! Soldiers don't get to choose. And you're at the mercy of the CINC (& advisors) who change every 4 years. Or get stuck with the last guy's shit, so other changes suck while people figure out better ways to eat a shit sandwich.

I'm with ya policy-wise - wanted to get on a plane to Ukraine in March, except too crippled & fucked up because of invading Iraq. Even w/training & experience, would 100% get someone killed.

The irony being, I joined for what seemed like a noble cause ATT: peacekeeping.
Would've gladly gone to Afghanistan after 9/11, right or wrong.
But no, got sent to invade a country that had nothing to do with that. Yeah it toppled a dictator, but that destabilized the whole region & enabled the rise of shit like ISIS.
Which was fucking predictable to anyone who'd studied the region w/o drinking ~neocon koolaid. But they DGAF & it felt wrong leaving my guys to whatever trigger-happy knucklehead might lead them if I fucked off.

\soapbox

The point is, yeah it's complicated, and many troops deserve a shitload of credit for sacrifice & selflessness... but military has its problems too. If it'd really bother you to serve in an unjust war, srsly better to not join!
You seem like one who'll try to serve the common good in other ways, including being a decent & thoughtful person. Which is awesome :)