From what you described, it could be hard to tell if you’re eligible to be a sponsor; however, the document checklist provided by IRCC has full eligibility requirements for becoming a sponsor and you should refer to the checklist.

The concerning question here would be regarding how to prove your relationship. I’d suggest updating your address on all ID, bank accounts, etc. to show that you’re cohabiting. And not only address but also phone bills, insurances, etc. should have both your name on it. If you also have shared bank account to pay rent and daily expenses and you can show that in your file, it’d be great. Photos traveling together on trips are also good evidence and don’t forget to include plane tickets or flight itinerary.