I think if an adult is “age regressing” unconsciously then it’s caused by a mental disorder - schizophrenia, dissociative state, personality disorder or dementia.
I doubt that is the case here?
Age regression is quite common in children and usually a result of stress, trauma or jealousy of a younger sibling.
In children that can be managed with care, love and attention and essentially ignoring or not engaging with the “baby talk” or behaviours such as tantrums or whining, babbling etc.
A child then grows out of the habit and learns more mature ways to communicate and gain attention.
In adults it’s different and the cause needs to be established and treated.
Behaving like a baby (on purpose) is a defence mechanism- a way to avoid adult emotions and adult interaction. Possibly as a way to cope with deep insecurities or fears, or PTSD.
Op- I think you’re trying to “resolve” this issue by adopting the approach you would with a child; you’re giving your partner care, love and understanding and attempting to ignore and not engage when they’re in baby mode. You’re trying not to “reward” the behaviour.
However, your partner is an adult, they won’t “grow out” of this, even if you insist they avoid doing it: (you’re calling this your boundary)
Quite frankly, they have no intention of addressing the issues causing them to behave like a toddler.
They are putting their enjoyment of acting like a baby as priority.
This is all under a guise of “this is ME- you have to accept it because you love ME”
Red flags all over that one!
Do not fall into the trap of thinking you can be a saviour here. I’ve no doubt you feel deeply sympathetic of the issues you’re partner has and want to approach this kindly.
However, you’re entering into an extremely unhealthy relationship dynamic by doing this.