I struggle with reciting prayer in my head. I can say it out loud but when it comes to saying it in my head it is very hard. I can't explain why. It just is. It will take me almost an hour to do it sometimes. What should I do if I go to the mosque and read with the Imam because I cannot keep up with him and I am often behind? When everyone stands up I am still down. It is very hard for me. What should I do?
In most madhabs the Imam's recitation is enough (and this seems the most proper view). That means if the Imam recites loudly you don't need to recite, you just have to follow/listen to his words (see for example this hadith and this one)... so far for the Qur'an recitation in case of jahr ("loud") prayers or prayer-parts.
Now in silent prayers try to recite only short surahs after the fatiha. You must do your best to recite at least al-fatiha as it is considered as fard/farz by most madhabs. So if your Imam made takbir to transit to ruku' you should just end the recitation of the verse you are at and follow him (avoid doing this in the recitation of al-fatiha unless you don't expect to catch or keep up with the imam again, see the hints at the end of my post).
When it comes to sujud, ruku' etc. what you necessary need is saying the words/dua's at least once: subhana raby-al-'Adhim (wa bihamdih) (during ruku', saying it three times is recommended) and rabana (wa) laka-l-Hamd (when you stand up from ruku') and subhana raby-al-'Alaa (wa bihamdih) (during sujud, saying it three times or any du'a is recommended). Note that saying the words with "wa bihamdih" in ruku' and sujud is optional.
So as you see from this point of view the major parts of prayer could be done easily unless you'll having some kind of learning disability as Darth Vader is pointing at. In that case I'm pretty sure that your efforts would be considered by Allah as HE say's:
Allah does not charge a soul except [with that within] its capacity. ... (2:286)
General hints: The Imam leads the prayer, so he is "responsible" for the congregation. You have to do your best to say at least the minimum necessary of the words of the prayer, if the imam moves to an other "position" you should follow him as soon as possible. Try to avoid being more than one position behind: For example Imam does 2nd sajda while you are in the first. If the Imam is praying too fast try to let him know.
See also this fatwa on following the imam in prayer.
And Allah knows best!
Have you ever thought you may have Auditory Processing Disorder? This is a learning disability that makes it hard to process sound. It's not a problem with hearing, its just a problem with how the brain perceives the auditory data. If my assumption is right, then you may want to look into this and maybe inform your Imam this, then that way you can adjust how you are receiving the information.
For me I have dyslexia, and I cannot read the Qur'an. I need it read to me for me to understand it. It doesn't mean that I am dumb, it just means that I process information differently.
Are you a muqallid or Ghaire Muqallid? If you are Muqallid, then, In Hanafi Madhab, Imam's recitation is enough for those who are attending the prayer behind him.
If you are not, then, you'd better try to imitate what Imam has to recite with utmost sincere intention.
And it is intention that will be counted in the judgement day, if Almost all resorts are exhausted.
So, I'll suggest you try at your best if you are not muqallid of Hanafi Fiqh.
If not, then, you do not need to recite because Imam's recitation will suffice for the followers.
And Allaah ta'ala knows best.