Assalaamu alaykum. I have difficulty pronouncing the Athkaar (pl. of Thikr [expressions of remembrance of Allaah]) silently. Therefore, most times, when I pray behind the imam, I am unable to complete the recitation of the Athkhaar before he moves on to the next position. That said, when this occurs, I sometimes try to force the recitation of the Athkhaar so that I do not miss the position which he just entered into. At other times, when my tongue becomes completely tied, I follow the imam directly. When my tongue gets tied like in the above mentioned scenerio, what should I do?

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. 

Having your tongue getting tied up is a result of obsessive whispers (Waswasah) so you must totally ignore them and not get carried away with them. Try to resist these doubts as much as you can until Allah, The Almighty, rids you of them. Until then, you may act according to the concession given by some scholars who are of the view that it is not obligatory on a person led in prayer to recite Al-Faatihah (the imaam's recitation is enough) and the view of those who consider that the Tasbeeh (saying: Subhaan Allah [glory be to Allah]) said in the bowing and prostration is a Sunnah and not an obligation; by doing so, you will no longer feel any difficulty.

Hence, if you cannot mention the Athkaar due to those obsessive whispers, then just follow your imaam and your prayer is valid, Allah willing. However, you must strive to repel these whispers and try to say the Athkaar in a normal, natural manner without overburdening yourself.

We ask Allah to grant you safety from every evil.

Allah knows best.