I've been living in Korea for a while and I suspect I've been making social mistakes without realizing it?

Nice self-awareness, and honestly locals forgive foreigners a lot, but a few adjustments earn real respect. The big everyday ones: always take your shoes off entering a home (and many traditional restaurants and guesthouses), and use two hands when giving or receiving something to someone older or senior, whether money, a card, a gift, or a drink, since one hand can read as dismissive. At meals, do not stick chopsticks upright in your rice (it echoes a funeral rite), do not start eating before the eldest person does, and when drinking, pour for others rather than yourself, hold the bottle with two hands for seniors, and turn your head slightly away when you drink in front of someone senior. Tipping is not customary, so skip it.

Hierarchy and age shape a lot: people may ask your age early so they know how to speak to you, which is normal, not rude. On transit, keep calls and conversation quiet, and leave the priority seats for the elderly, pregnant, and disabled even when empty. Blowing your nose loudly at the table is frowned on, so step away, and writing someone's name in red ink is a small taboo. In work and social settings, being a bit indirect and reading the room (눈치) matters more than blunt honesty, and politely declining once or twice before accepting is common. None of these are dealbreakers, but taking shoes off, using two hands, and respecting the drinking and seniority etiquette covers most situations, and your friends will quietly notice. When unsure, copying what the youngest Korean at the table does is a reliable shortcut.