I agree with Alex and Yukala. You are putting way to much pressure on yourself and hence making any PR counterproductive.
You need to start practicing with other books that are not a pressured "need", it will boost your confidence that you can PR and make it easier to eventually use the process with PR your technical stuff.
I could suggest you Photoread/Photofocus on all those Chemistry charts and if you use flashcards, Photoread/Photofocus flipping through those...stating your purpose sub-vocally and chanting while you do it. Do that while you're relaxed...like when you're waiting in line for something or taking a break between classes. But don't give it such high importance that you get anxious. Take those cards with you everywhere....photoread them...put them away...talk to your friends or do something else...photoread them again whenever you have a minute to spare. Just do that process over and over again, passively doing it, thinking "I'll get it". You might eventually get an urge to Mindmap something, dream about it, suddenly have clarity to your studies. But don't force it. Listen to your inner ques or urges to open to a certain page or chart, it's your subconscious telling you something important in your studies.
PR is done in layers...but not every step is needed for every type of PR.
I have the home study course...I took the 7th CD and used it as guidance going through the process. I keep doing it until I stopped needing it and go with how I FEEL best fits my particular reading.
Lastly, take into consideration what type of learner you are. Are you mostly visual, auditory or kinetic...this could be part of why you are struggling. You may be so focused on being "visual" and if you are kinetic in nature...you missed out on connecting things together. Mindmapping is good for kinetics and visual. If you're auditory...then perhaps explaining what you've learned or talking to yourself aloud while pondering your studies could help. If you're auditory and you mindmapped something...connect those pictures and words and turn it into a rap song or kids song. Sounds silly, but I bet you wouldn't forget it! If you like computers and typing...they have tons of free mind mapping programs online...you could cut and paste visuals and what ever else would help. You could buy a huge posterboard, mindmap chemistry thought and stick it where you walk by all the time...your subconscious sees it (think "dream boards") Be creative and even silly if you have to. You gain trust in your subconscious when you let the results of PR happen.
Good luck...I know you can do it!!!
