That is not a good idea. Every time you enter Canada, you are assessed on your likeliness of complying with your temporary status as a visitor. If it seems to CBSA that you are trying to "live" in Canada as a temporary visitor, they can limit or refuse your entry.

You can apply online to extend your stay without leaving the country before your six months elapses, but the same thing applies: you must explain adequately why you need to extend your stay as a temporary visitor.

Separate from that, if you and your girlfriend become common-law partners or married, she could be eligible to sponsor you to become a permanent resident. Staying temporarily during that process still applies, but if approved the end result would be that you can stay in Canada permanently and be able to live and work there freely.