The mental part.
I think you really need to imagine being huge or becoming bigger. If you do not believe in the exercises and all the time doubt that it will have an effect, you are bound to fail.
To me it is very important to visualize what is happening in the penis during the workout and to see, as an inner picture, the growing of the tissues.
It is also important to start feeling bigger. I was only 1 week into peeing when I first started to image myself huge when having sex. It really works (until you turn on the lights :-) and gives me much more pleasure from sex.