…they think the abuser is just ‘troubled’ or ‘had a bad childhood’ and that the relationship ‘just has its ups and downs’.
So they misread information that is not for their situation and misapply it, making the abuse worse.
(credit u/greenlizardhands)
Healthy relationships don’t have ‘ups and downs’, generally speaking, because everyone within the relationship respects the other person’s boundaries. So even if you’re feeling sad or upset or angry, you know you don’t have the ‘right’ to take it out on someone or destroy their things, etc.
Abusive or toxic relationships end up with arguing over reality
…whether someone’s feelings are right or wrong, whether their opinion/belief/ideas are right or wrong, or their actions; and the person who has decided they are the judge, jury, and executioner is the person who has decided the other person has to change their mind or actions.
(Versus a healthy person realizing that they are not compatible with this person on a significant issue, and therefore ending the relationship.)
That person – the one acting as the arbiter of what is right and wrong – may even use tools for healthy relationships to browbeat their significant other into changing their mind: so the tool for a healthy relationship itself is even used in an unhealthy way.
Healthy relationships are relationships where each person’s natural, reasonable boundaries are respected.
Healthy relationships are not controlling, and healthy people do not want to control others. In my experience, they tend to back away from ‘messy’ situations, not try and control others or try and educate someone else that they are being controlling. They tend to honor their discomfort with the whole situation and back away, like you would from a venomous snake.
Advice for healthy relationships will never work for unhealthy ones, because that advice assumes a foundation of respect between reasonable people who are actually compatible and agree on reality.
And victims fall into the trap of mis-directing what they read: grace and compassion for the abuser instead of themselves; binding rules and credos for themselves and never the abuser.
…because they’ve unwittingly accepted the abuser’s (false) reality as real
and the image the abuser reflects back to them as their own.

A great example of good advice for good relationships that is bad advice for bad relationships? “All relationships require compromise.” In a healthy relationship, founded on mutual respect, between reasonable people – yes! Compromise is good, and both people will compromise, for example, like on what restaurant to go to.
Compromise in a bad relationship is bad. Because you’re compromising your safety, your boundaries, and who you are as a person. You’re compromising your sense of reality. And the other person is taking your compromise as permission to demand more…because you’re the only one compromising.