What is the ruling regarding my previous prayers, and do I have to repeat them, given that I used to make mistakes in the Tashahhud by adding "lillahi ta'ala (for Allah Almighty)" after "at-tayyibat (good things)," and by emphasizing the "nun" in "an la ilaha illa Allah (that there is no god but Allah)"?

An incorrect recitation of the tashahhud does not necessitate repeating the prayer. If the worshiper alters the tashahhud in a way that invalidates its meaning, their prayer is not valid. If they do not alter it, the madhhab (school of thought) holds that the prayer is valid. A lisp (or error in pronunciation) in the shaddah (doubling of the consonant) of the letter nun in "Ashhadu an la ilaha illa Allah" (I bear witness that there is no god but Allah) does not invalidate the prayer nor necessitate its repetition. Therefore, you are not required to repeat past prayers. However, you are obliged to pronounce the correct words in the future.