Not to mention the fact that / (On top of this) I wrote these sentences: The absence of an environment conducive to English learning severely limited my progress -- the lack of English media and friends to chat with, and the environment in general, were the biggest barriers. Not to mention the fact that / (On top of this), I had to work 10 hours during the day and after work I have limited time to practice English. Shall I use "Not to mention the fact that" or "On top of this". They are the same meaning?