Do my prayers become invalid because I am unable to pronounce some letters correctly, which affects the opening takbir and the rest of the prayer?

If the mispronunciation (laḥn) in the takbir (Allahu Akbar) is overwhelming for you and persists in the rest of your speech, and you cannot overcome it through learning, then all your prayers are valid, and nothing is required of you.

It is stated in Al-Majmu' by Al-Nawawi: "If the person who mispronounces (al-art) or lisps (al-althagh) was able to learn but did not, his prayer is invalid in itself, and it is not permissible to follow him [in prayer], without disagreement. However, if he was unable [to learn], either because his tongue does not obey him, or because the time was too short and he could not learn before that, then his prayer is valid in itself."

We warn you against falling into [religious] whispers (waswas), for their harm is great. If the matter is nothing more than mere waswas, then know that turning away from them is the most effective cure for them.