Okay - the gusts are intentionally modeled that way.
Admittedly I've never managed to find a measured time series of gusty winds with a good resolution, so I've used the following reasoning:
What happens in reality is that you move through a windfield with vortices and eddies, these are small-scale phenomena (out of my head, the funnel of a fully-developed Tornado can be 50 m across only, dust devils are some 10 m across), you're typically moving through them at 100 km/h at least (usually faster), so it takes you ~0.3 seconds to resolve the whole eddy. If you associate each gust with an eddy wall crossing, you'd get sudden wind changes in a fraction of a second.
Standing in gusty winds on a cliff facing the sea, that's certainly how it felt to me - there was no smoothness at all to the gusts.
You're usually not flying a glider in gusty winds, and on the one (uncomfortable) occasion I have done that, it was during a mass landing of all planes in the vicinity due to an approaching storm. So the experience should most definitely not feel comfortable in an ASK-13.
I just had my first flight in the real deal and had to continuously work on keeping it straight & level
Yes, that's one the things I've never really liked about YaSim - it feels unnaturally smooth, in reality you're busy with minute changes all the time. JSBSim does that for you, it has little to do with the weather as far as I know, it's how real planes behave.
Is the change when flying through changing winds really that abrupt?
I've been lifted from the seat till I hung in the safety belt and saw my lunchbox smashed against the canopy top by a sudden wind change in the mountains. It can be very violent, it can also be very smooth. METAR reports of varying winds usually mean the gusty type as far as I know, which is why you get hard, short-timescale variations when you try to set wind variation in the METAR.
When you ask about "wind mode", what exactly do you mean?
There's various modes to set the wind, starting from a constant overall wind to a 3-d interpolation over a pre-specified windfield. But since you're specifically referring to gusts, that's not relevant here.
By the way, maybe there should be some variation in the wind anyway?
Maybe, but smooth variations you wouldn't notice anyway - and you do get wind variation by the boundary layer effect close to the ground, by the altitude variation as you rise and by the spatial interpolation as you change position - which you didn't seem to notice so far, thus making my point...
and then massively reduce the frequency of gusts but keep them instantaneous.
Based on what? If you manage to dig up actual time series of gusty winds so that we have some good data to model them, any time. Otherwise it's your word and experience that it doesn't feel right against my experience and semi-quantitative reasoning that it does.
(you can adjust gust frequency user-side by the way, since it's not reported, I added that freedom....)