I think Motzei Shabbat is hard because
We can actually be quite tired on Shabbat as the week catches up with us, and also from making a lot of conversation.
Shabbat is ordered, with commitments to go to shul, and to make sure we eat three meals with Cholla, and so on.
And some people find it hard to cope with NOT working. Work can be a distraction from our lust, and it can be very bound up with our self-esteem, but remove work from our lives, remove us from our daily routine, and it presents a challenge.
We can feel that we have no way to make "progress", and so our esteem suffers.
So after Shabbat it is a bit of a release, and we feel a bit of "freedom", and give in to lust, and might feel we need to seek a pleasure hit to improve our low esteem.
I know Shabbat is meant to be the opposite of this, but it can feel different from how it is supposed to feel.