One more point is not to evaluate yourself during Shabbos, asking yourself what you are presently feeling. Sometimes after the fact it is easier to look back and realize how wonderful it was. Focus rather on how to utilize this holy time, and that makes the Kedusha a part of you. Hashem gives up these feelings sometimes to show us what we can reach. You didn’t falter by not retaining it – it was a gift of being to gain a real live understanding of what levels there are to attain.
May your life be filled with Kedusha.