Ukrainian: A Real Mother Tongue?

It happens occasionally that I'll have a little slip and Ukrainian will be about to come out of my mouth, but my tongue catches it before I confuse the daylights out of the people around me. It's quite infrequent as my English has dominated, and is considerably better than my Ukrainian at the present time. In fact, I almost never use my Ukrainian except when speaking with my mother these days. So, during a discussion where a friend and colleague was explaining how in a certain context when she only uses Spanish, she reverts to English solely when she is quite angry or frustrated. I thought about this and realized that all of the slips that I can recall are when I am around children (a regular occurence in my current field) and I want to yell at them I have to stop myself from saying it in Ukrainian. When I was a child, Ukrainian was much more common for me, and it was not uncommon at all for me to be commanded in the way I seem to be wanting to do deep down to these kids now. But the real question is, why?

My only theory is that there is a typle of conceptual "slot" for certain grammatical forms that each person has inside. In English, most of the forms just default back to the root, so for example the verb PUSH has the same form for many different tenses (I push, you push, we push, you all push, they push) and the imperative mood (push!), while in Ukrainian, there is a unique form for the imperative mood, пхай¹, and this form seems to occupy the default for the imperative. Of course, for this to be true for this reason alone, I'd probably have to default on other distinctions in form, such as tense, that exist in Ukrainian and not in English. Most likely, this is not the case, but I wouldn't doubt it as being a significant factor.

Incidentally, the reason I chose пхай was because the only instance I know of my mother unknowingly slipping out of English and into Ukrainian was when helping a small child with his shoe and wanting him to push his foot into the shoe.

Maybe Ukrainian is just naturally suited for guiding young ones. I suppose that is just as plausible as anything else here.

Note¹: пхай might not be best translated as push in all contexts (it's also the word for pick as in don't pick your nose!), but it's certainly the one to use for when you are telling somebody to shove their foot into a shoe.