My first advice is to either, remove the humble opinion part or rephrase it.

Why?

This is an official correspondence, right? Your opinions generally should not have a place in those.

You see, problem with opinions is that they can get misinterpreted. A lot. And often on purpose. So, someone reading that might get an impression that you are boasting about your abilities; someone might get an impression that you are being too 'clingy'. Remember; facts only. You lay down the facts and hope for the best. Maybe 'hint' at the fact that you are a victim of inconvenience; not of your incompetence (even doing this is pushing it) If they ask for your opinion -- give it.

My second advice is to show some willingness to prove yourself.

Like, "I am available on Skype if you wish to dismiss any doubts, you might have, about my English speaking level."