Masha' Allah sister; appreciate your enormous effort to follow the Sunnah and pray the way the Prophet ﷺ used to pray! But al tawarruk/iftirash are Sunnah, and are therefore not obligatory, but have the extra reward of being the Sunnah.
However, one must prioritize his concentration (khoshou') in Salat as your reward in Salaht is proportional to amount of concentration you have in it, and one must contemplate that he's in front of his Lord Allah Subhanahu Wa Ta'la. Moreover, Islam always preserves the health of the individual and hence any action (intentionally) that leads to self harm is not permissible.
I also would like to bring your attention this is mighty Hadith of the Prophet ﷺ where he said: "The reward of deeds depends upon the intention and every person will get the reward according to what he has intended." So sister if your intention was sincere in following the Sunnah and you can't practice due to health issues but intended to, then insha' Allah you will be rewarded the same reward (Ajr) according to that Hadith even if can't practice it!