What I miss most is French. Learning another language with all kinds of different expressions is great for the mind, allowing you to access hidden tools lying dormant in wait. There was a purpose, a goal, other than earn money etc. As I started to understand non-Native French speakers I felt this uplifting cathartic release, as if I was accomplishing something, learning. I figure that unless you're extremely gifted it takes 2 to 3 years to be able to somewhat fluently express yourself in a variety of different situations and a couple more to effectively work in colloquial expressions with the correct timing and accent. If you can understand a sentence which synthesizes three domains simultaneously (an old regional phrase popularized once more for 2 years and remembered by those aged 28 to 32, a famous line from a forgotten hit single, and the translation of a phrase worked into the language from a commensurate dialect) you are succeeding. Like a giant hole, not learning French anymore, which will be difficult to fill.