As a quick example, the link grammar immediately highlighted that your second sentence:

  > I have been living in London for past 6 years
  > and my English still sucks.

doesn't parse. You need a "the" before "past" for natural sounding English. It really might help a lot, and perhaps get your error rate down to an acceptable level without having to pay for external assistance.

As a footnote, in the fourth sentence it did not pick up that you meant "conversation" and not "conversion."