Keep in mind that when doing a workout in erg-mode the trainer resistance is controlled by an inverse combination of how hard you are riding and the prescribed wattage output. That is, in erg mode as you pedal faster or shift to a bigger gear at a constant cadence the trainer resistance will decrease to keep your wattage output at the desired level. The opposite is true if you lower cadence or shift to a smaller gear at the same cadence.
In sim-mode, however (presuming you are on a smart trainer), the trainer resistance is determined by the terrain. Steeper = more resistance than flat. And at the end of a workout your trainer will switch from erg-mode back to sim-mode, and the resistance will change from what the workout wanted your output to be (which is probably about 50% of your FTP at the end of the workout) and what is required for the specific terrain you are on.
So, for example, if you happen to be doing a workout on the Alpe, as soon as your workout ends the trainer resistance likely increases by A LOT if you’re still headed up, or goes to something close to zero if you happen to be headed back down at the time.
And with the erg-mode trainer resistance being inversely related to the speed at which you are turning the rear wheel, if you are doing a workout in a 53 x 12, again let’s say you’re heading UP the Alpe, the trainer resistance is VERY slight. And when the workout ends, and you revert to SIM mode, you can imagine what it would feel like if you’re climbing a grade like that IRL in a 53 x 12! The amount of watts you would have to put out to keep pedaling at a reasonable cadence would be enormous.
So, my advice to you would be to choose a small-ish gear for your workouts and spin at a cadence that is comfortable for you. The trainer will adjust the resistance as necessary. (Your avatar will move at the same speed in-game for a specific wattage on a specific terrain no matter what gear you are in.) And when the workout is over you will revert to sim-mode, riding a small gear at a comfortable cadence. You’re certainly likely to still feel the change, especially if you happen to be heading uphill at the end of the workout, but it shouldn’t be too drastic. From there you can find the right gear to ride in for the rest of your free-ride.
I hope that helps at least a bit.