All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The fact that you do not articulate the Ghunnah properly and saying it at the wrong place does not invalidate your prayer. Reciting according to the rules of Tajweed known today is not obligatory, and breaching them does not invalidate the prayer.
Fataawa Noor-un ‘ala ad-Darb by Shaykh Ibn Uthaymeen :
“Reciting [the Quran] with Tajweed according to the well-known rules is only a recommended and perfect way. As for the obligation, it is to articulate the letters as they are in a way that one does not omit a letter or add a letter. As regards the characteristics of letters, such as the Madd, Ghunnah, Qalqalah, and the like, then this is only recommended and not obligatory.
Allah knows best.