Upvote:3
What matters here is the Visa Waiver Program (VWP), not ESTA. If you are legally resident in Canada, which I presume is the case if you're working there, you are exempt from the usual VWP requirement for an onward tickets out of the US and the "adjacent countries/islands" rule when counting length of stay in the US. (You still can't stay in the US for more than 90 days at a time.) Instead this applies:
The burden of proof that the Canadian citizen is not an intended immigrant (plans to make the U.S. their primary residence) is always on the applicant. There is no set period of time Canadians must wait to reenter the U.S. after the end of their stay, but if it appears to the CBP Officer that the person applying for entry is spending more time over-all in the U.S. than in Canada, it will be up to the traveler to prove to the officer that they are not de-facto U.S. residents. One of the ways to do this is demonstrate significant ties to their home country, including proof of employment, residency, etc.
If you are working for a Canadian employer, paid in Canadian dollars, renting a place to live in Canada and spending all your weekdays in Canada (= "more time over-all"), you should easily qualify. Bringing documentation to prove this (Canadian work visa, contract, pay slips, bank statements, rental agreements, etc) would be prudent.
Disclaimer: As always on this site, this is not legal advice, and CBP has wide powers of discretion if they suspect that you're working in the US illegally on weekends or planning to move in with a significant other. I would advise you to mix it up and take turns to visit each other, instead of going to the US like clockwork every single weekend.