My English language skills have reached a plateau, what can I do to improve further?

Watching TV, working and talking to your friends may not be pushing your language abilities fully. Especially if you are simply watching popular TV. If you want to improve your fitness, you have exercise to the correct level. Fail to do enough and you lose fitness. Likewise for learning English, you have to push yourself beyond your comfort zone to make improvements.

My suggestion would be to deliberately push your language use into specialist academic areas by studying them. For example, reading up on a science textbooks/journals, buying financial or broadsheet newspapers rather than regular newspapers, watching more documentaries and less soap operas, and generally engaging in higher level discussions with your GF and work mates.

Don't expect quick progress. A score of 8 on IELTS is very high and 9 would be perfect native. For 9 we are saying that the examiner thinks you are Canadian and well schooled too.