We are using present tense with depuis as in depuis que je suis toute petite where as we are using imparfait in the sentence mon choix était fait ....

You can find some of that hand-waving here, but basically it is just "that's the way it is".

https://french.kwiziq.com/questions/view/depuis-que-present-tense

Yes, that's correct. If you ask why, one can wave his hands attempting to explain it, but what it really boils down to is: that's just the way it is. ;)