This isn’t a hardtail issue. Or tire pressure. Or any of that stuff.
Basically, you’re finding the seats uncomfortable. There are several possibilities and I’ll outline them below.
-The seat is too high or too low, along the same lines it could be too far forward or back, or the nose too high or too low. Fiddling around is in order. With your cycling shoes on. This is ideally done in a stationary trainer mount.
-Your butt isn’t yet conditioned. Though it probably is, since you mentioned road bikes too.
-Possible you left the padded shorts at home? Just cause mountain bikers wear baggies doesn’t mean there aren’t pads underneath.
-Are you descending on your seat? Allowing your taint to act as a shock absorber? Don’t do that, get a dropper and get off the seat for the gnar. It hurts less and improves what you can get through. I figure my hardtail has nearly a foot of rear travel when I’m standing with the seat dropped. Legs are useful for much more than pedaling.
-Finally, some seats just don’t fit your body. You might have to try several. I’d start with something similar to the one on your road bike but be aware that your riding position is a bit different so it’ll feel different.