Yes, there are multiple ways to overcome this.
1. Tabluate your thoughts/ideas and memorise them - Memorise your KEY ideas germane to thematic concerns within your essay
2. Memorise a list of quotes, to validate your ideas (This is not only logical, but ensures cohesiveness)
3. USE STEEL method; Statement, Technique, Evidence, Effect and Link
PLAN!! - Planning during reading time in your head and jotting down quick points/notes after reading time will definitely aid you in overcoming this problem - Ensure that your notes are coherent and logical.. AND that they answer the question - PLANNING is essential for success in English.
As time progresses, you will definitely be able to write on the spot. For the time being, inculcate these practices to ensure a coherent, sophisticated and structured essay
Hope this helps!! If you need any help or have any questions, you can always ask me
Ps: Sorry for the rushed response and myriad typing errors it may contain//