Only you can decide if your situation is fair.
I moved in with my sweetie 12 years ago, and initially I paid $350 a month. His mortgage, at the time, was in the $1100 range, so I was paying about 1/3. He makes double my income, fwiw. Later he built onto the house, refinanced, and his mortgage is about $2000 a month now and I pay $500 a month. I do buy half the groceries and we split chores, but admittedly he has a larger portion of the bills.
"A mistake is not failure but feedback. -Rod Gilbert"