I think that being roommates or housemates is the most difficult part of any couple or marriage. I'm writing from 30 years experience with my wife, who, earlier on, was my girlfriend I lived with.
The first thing is to separate the roommate/housemate issues, the financial issues, and the relationship issues. Then deal with each one. (Relationships are a lot of work!)
Roommate/Housemate issues: Are each of you at home and comfortable in the space? Are you also comfortable together in the space? Do you each have the right amount of time in good space alone for your own activities? And do you have good times together at home? If the answer to any of these is "no," then, as Captain Picard would say, "Make it so!"
On financial issues: Minikitten offers one good solution, separate finances. There are others. But if you can't come up with a clear joint arrangement, separate finances is best.
On relationship issues: See my hubs about renewing a marriage with daily gratitude, the 5 Love Languages, and the 5 Languages of Apology. They all work for unmarried couples as well. And they are the best and simplest tools I've found in 30 years of improving a relationship, seeking great tools, and being a relationship coach.
Lots of work, but well worth it for the joy!