Hi everyone!
<I’m not a native English speaker so I’m sorry for any mistakes>
Could you help me guys with CORRECT translation of ‘My Princess’ (to Latin of course). I hesitate between these options:
‘Mea Regina’
‘Mea Domina’
‘Mea Principem’
Which one would be the best? Which is correct and proper?
Thank you in advance! 
None of the above is right. I am going to give you two possible translations, but you should keep in mind that Latin is an inflected language, meaning that words change their endings depending on the grammatical role they play in the sentence. Much like you wouldn’t say “give it to I” in English but rather “give it to me.”
That being said, titles are notoriously difficult to translate. It is much easier to find a Latin title and understand it in modern languages than vice-versa; different conventions would be in use in different countries at different times. Two possible ways of translating “my princess” would be “regula mea” and “principissa mea”, but really, if you’re trying to impress someone maybe picking a language other than Latin would be a good idea.
If anything, because our modern convention of calling someone a “princess” as a term of endearment has no tracing back to Latin poetical conventions that I know of. If ever a Medieval or Renaissance speaker of Latin were to hear you call someone “My princess”, I bet their first thought wouldn’t be, “How sweet, how did you two meet?”, but rather instead something like, “So you swore an oath of fealty to her father?” (Which might be the case. What do I know.)
I am so grateful that you explained it to me. Thank you.
I just want to engrave that phrase (‘My Princess’ in Latin) on a bracelet as a gift for my girlfriend. But, you know, I want to do that perfectly correct.
So in your opinion, Anphph, what would be better in that case? ‘Regula mea’ or ‘Principissa mea’? 
I wonder in what contexts regula might be used for princess? Princeps is m/f and I know that as a substantive it is normally masculine but its a possibility. How about Imperatrix, she who commands. If you want something more endearing Catullus uses mea puella.
As anphph says what might seem like a title to us is loaded with an ideology either of power or sentiment. There is no “correct” translation. You have to be a bit clearer on what you mean by “princess”.
I hope this doesn’t seem unhelpful.
As seneca said, I would discourage the use of either, and would instead opt for a term that’s in use in the love literature. “Mea puella” would be an option; in these cases the words usually go in the dative, meaning the form that implies the ‘indirect object’ (==“to my girlfriend”), which in this case would be “meæ puellæ/meae puellae”, if you’re willing to choose that version.
Thank you seneca2008 and anphph for answers and help. I see your point.
You encouraged me to choose the ‘love literature term’ and it will be that one from Catullus probably (with ‘puella’). And thanks for correcting my original ‘translations’ of this phrase 
