Venlafaxine (SNRI).
Took a couple weeks to settle into side effects, and about half a year to dial up the dosage (with side effects being a few days to a week after dosage changes).

But it was pretty immediate. I think a big part was the act of treating it, the act of getting help.
Had a lot of ups & downs, took a while to "trust" it, to recognise it working. And building the habit of taking meds helped maintain a schedule.