I (14 years old) have just found out about the 14 pillars of prayer, and I know I can do 13 of them but one of them I don't know. I have read that not doing one of them can invalidate the prayer. I don't know how to do the dua in the last rakat (allah-umma sali a'la...). Does this mean all the prayers I ever did were invalid? I feel like I am going to end up in Jahannam and that I have just wasted my life.
The major point is if you didn't know a ruling before you must learn it from now on and do your best to perform it correctly.
In most cases scholar would say as you did it as good as you could and to the best of your knowledge, your prayers would be accepted, some would recommend performing some nafl (sunnah) prayers to gain the possibly lost thawab.
If you didn't know the exact wording of a du'a or at-tahiyat or salawat that is neither a sin nor have you missed a fard (obligatory or essential act) as the fard if it was one is the sitting for the tashahud not the (exact) words you say! There are a few forms which have been passed on by the sunnah of our Prophet (Allahs blessings and peace be upon him) learn (at least one of) them if you can't memorize them write them on a paper and put it on the ground and read them from it when you need to perform tashahud.
It is always the best to perform a du'a as it was passed on by sunnah, but in case one can't memorize one is free to say any du'a yes according shafi'i school of fiqh (and apparently the hanbali too, at least for the last tashahud) it is essential to say salawat on our Messenger and his family as far as I recall but this is an opinion from a mujtahid, other fiqh schools have other opinions. So if you missed an essential part according shafi'i school doesn't mean that you missed it according any other school and as it is a matter of ijtihad, so each party has evidences and if the opinion of the shafi'i school was much stronger then I'd say good and upright scholars would advocate it and impose it to their school.
A major reason why it can't be a fard is this sahih hadith, which you also may find in sahih Muslim, sunan abi Dawod and sunan an-Nasai. Where our Messenger taught his companions how to recite the prayers and blessings upon him and he didn't emphasize on the importance of saying it, but only said "say" instead of saying something like "you must" or "you should" ... so the most precise view seems to be that this is a(n) (emphasized) sunnah, maybe it could even be considered as a wajib which means one is asked to do it if he can and know the formulation.
See for example this fatwa on the ruling of sending blessing upon the Prophet in the tashahud.
Here are links for formulations of tashahud which have been passed on by some sahaba -so I refer to them when "naming" a form of tashahud- from sahih al-Bukahri (tashahud ibn Masu'd), sahih Muslim and sunan an-Nasai (tashahud ibn 'Abas), sunan abi Dawod and also quoted by ad-Daraqotni (tahsahud ibn 'Omar), sunan an-Nasai, sunan ibn Majah, sahih Muslim and sunan abi Dawod (tashuahud abu Musa al-Ash'ari), and here some of those formulations with a transliteration from al-Muwatta' 1, 2, 'Aisha using tahsahud ibn Masu'd 3 & 4 as I'm not sure whether you need it too.