What might be equivalent to "living paycheck to paycheck" , on an emotional level?

Here are two that immediately come to mind:

living in the moment
emotionally unstable

emotionally labile

The classic book about Borderline Personality Disorder, which is characterized by chaotic relationships, black-or-white thinking, and extreme emotional instability is: I Hate You, Don’t Leave Me, which I think is a good description of the phenomenon you’re describing.

Serial monogamy. My ex husband demonstrated this all too well. Having a relationship was important, the “who with” was an afterthought.

Ouch, Inara.
Glad there’s an ‘ex’ in there.
Great strength- good for you!
:)

Monogamy Cereal – Bland and tasteless but good for you.

Hanging on with your fingernails. (at least that’s how it feels to me, and I deal with it a lot)

It’s usually overstimulation and/or the utility of coping skills. You can only have so much of each.

Running from bed to bed and person to person, but nothing ever sticking.

^^ If you were running from bed to bed and person to person, there would be some stickiness involved. just sayin’