^well an upshift from 1st gear to 2nd gear is the largest change in gear ratio you can put the box through entirely, at least that you'd regularly use (shifting into 1st while moving is actually the biggest, but you rarely need to do that, or ridiculous stuff like completely skipping gears)...more so than downshifting at even crazy revs...this is exactly why most manual gear boxes have double or triple cone reinforced synchro's only on the 2nd gear collar...
you can make both upshifts and downshifts helplessly horrid on gearbox internals if you purposely do it...but in general, yes, downshifts are generally easier on the gear box...harder on the engine (gear box parts are slowing down, engine parts are speeding up)...but that is mostly related to how you drive...
whats likely happened is that the 2nd gear synchro has just been wearing down more and more from normal 1st to 2nd upshifts...with 'normal' synchro wear on 2nd gear, you'll notice it from 1st to 2nd only at first...but as it wears more and more, it'll stop properly synching even less aggressive ratio changes (i.e. 3rd to 2nd)...
you can slow wear as much as possible during deceleration by pulling out of 3rd, letting the clutch OUT, then precisely matching revs with light pressure on the shifter into 2nd...if you do it right, the collar will slip in with the clutch engaged (i.e. pedal out) and no fuss...but obviously that takes practice and a lot of paying attention...you can also just get in the habit of just skipping 2nd gear haha...luckily the FS has enough torque to lug around in 3rd pretty easily, just wind out 1st a little more before the shift...remember though, thats just going to put a s*** ton more stress on the 3rd gear synchro (MAJOR ratio change for that)...but it'll hold out until you simply find a cheap used gear box...