You have to use the language to learn it and retain it.

When I lived in Jordan I frequented Arabic lessons and had to use it in my day to day interactions with the locals, especially shopkeepers and taxi drivers.

I was never good at langauges at school but I found that using it daily it became second nature.
Since moving here 7 years ago I have not had to use Arabic once as I don't meet anyone who uses Arabic......as a result I have forgotten 99% of the Arabic I used to use.