Simply put, the dark side of caregiving is enmeshment.
An enmeshed relationship is one between two or more people in which personal boundaries are permeable and unclear. This often happens on an emotional level in which two people feel each other's emotions, and and over-concern for others leads to a loss of autonomous living. One does not have or no longer has a life with their own separate thoughts, feelings, beliefs, opinions, hobbies and so on.
Several online articles about this word help unpack it further.
There are at least 4 types of this dark side which may stand alone or even overlap one or two.
1. The eternal child.
They have been groomed by a parent to always respond like they are still the little girl or little boy, plus that is how the parent continues to see them and treat them despite all of the adult things they do in life.
Psychologically this is called infantalism. This is also a form of emotional abuse.
I have a relative who is still in bondage by this in her early 60's and somehow her marriage has lasted, but not well. She's been to therapy, but quit. She at times wants someone else to fight her war for her, but she will not fight.
2. The hurting child.
They seek to compensate for something that was absent from their childhood. They very often will endure abuse that not one else would in order to possibly see the parent become the loving, non-abusive parent that the never were. Sad to say, but they never will despite all presumptive hope that they will be the exception. We read plenty of this here.
3. The parent/child.
The overly responsible parent/child who is groomed emotionally to feel responsible for the parent almost as if there were their parent. That's called parentification and is also emotionally abusive.
4. The partner child.
This is called covert or emotional incest. This language is a hard pill to swallow, because we normally think of incest only sexually, but the partner child/parent is not sexual. However, it is emotional child abuse.
The partner child is when a parent makes a child their emotional partner either because the spouse is gone because of divorce or death.
Some do this with a child because they are not getting such emotional support from their spouse and it is easier to do this than deal with the marriage problems.
Very often in this relationship the parent will share things with the child that should never be said.
I have a very close relative that this happened to. Their same sex parent told them all about their sex like with their other parent. I'm surprised they got their own life and got married, but I'm glad they did.
In my opinion this is the absolute deepest and by far the hardest to get out of.
Those married to the partner child often feel like there is a third person in their marriage and it can get so bad as to also feel like being a single parent though married or basically single though married.