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, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
If you pronounce the letter 'Daal' correctly, then there is nothing wrong if it sounds like 'Taa’' to the one listening. However, if you sometimes pronounce it as 'Taa’', then you have to try to pronounce it correctly in Al-Fatihah if you are able to, otherwise, you are not blamed. If you can achieve this but it takes you a long time, then you still have to it if you pray alone. However, if you pray in congregation, then do not be late in following the Imam on account of your attempts to pronounce that letter correctly. Rather, you should follow the Imam, because the one who is led in prayer is not required to recite Al-Fatihah according to the opinion of the Hanafi and Maaliki scholars, who view that doing this is not obligatory for the one who is led in prayer. This opinion involves ease for you and those in similar conditions. You are not to be blamed for following such an opinion as long as you are unable to pronounce this letter correctly except by making many attempts that end up with you not following the Imam. This is so if you cannot actually pronounce this letter.
On the other hand, if this case is caused by whispers, then do not pay heed to them and act as if you really pronounce it in the correct manner from the first attempt. You can make sure that you pronounce it correctly by reciting the Quran before someone who is knowledgeable of the articulation points and the characteristics of the letters. If he tells you that your pronunciation of the letter is correct, then know that what you suffer from is but whispers that should be disregarded. However, if he says that the way in which you pronounce the letter is defective, then try to rectify it with the help of this person or someone else if that is possible. Otherwise, you incur no sin because Allah Almighty does not charge a soul except with what is within its capacity. In order to be sure, it is invalid for you to lead someone who is more efficient than you in the recitation of the Quran in prayer. However, it is valid to lead those whose level of efficiency is equal to or below yours in prayer.
Allah knows best.