Dear OP, I haven’t been precisely in the situation you describe but I’ve been in situations where I allowed my boundaries to stretch and those were awful situations. Nothing is worth it.
The following presumes that you’re concerned with either premarital sexual relations or temptations to it, or with how your limits of decency are stretched, how your conscience bugs you about blurring the lines between what is marriage and what is not. I am not presuming that you guys merely have two separate rooms in one house because you’re asking for advice to help you.
As long as your situation does violate what our faith teaches us about morals, God needs to come first, He says, in relatively modern language, “Seek first the Kingdom of God and its righteousness and the rest will be added to you,” (Matthew 6:33). We also pray for our daily bread without praying for the bread for the day after tomorrow. There are numerous verses referring to the fact that God provides for us. God cares for each sparrow that falls down from the sky, and we’re worth more than many sparrows (Matthew 10:31). Solomon himself, as rich as he was, wasn’t dressed nearly as well as a lily (Luke 12:27). In short, we need to trust God, trust ourselves to God, and do our job. This does require sacrifices sometimes but those are always for our good. It is difficult to believe that God will provide, it can be a huge leap of faith, nonetheless it is the right path. This can sometimes show already in this world rather than the next.
(For example one may feel like he’s losing a loving girlfriend, while in reality the sacrifice makes him avoid a nasty divorce, possibly without the ability to obtain a nullity decree, after a lot of pain, or a marriage to a person obsessing over sexual technique or performance or materialistic or not willing to accept rules which are of an external origin and/or which interfere with whatever that person feels like doing. Which additionally means losing the opportunity to be married to someone who actually does live the faith and sees things the same way. Such a sacrifice is already beneficial in this world, not only in the next.)
Perhaps it might help to look at the “rules” as more like the “righteousness” from that verse, i.e. not like a bunch of rules imposed by an eccentric landlord or 30 pages of small print from a service operator, but doing the right thing, basically doing what is just. The Catholic teaching is that commandments connecting to marriage (or what leads to marriage) are not mere ritual rules, they don’t deal with some kind of ritual cleanliness and purity, they tie into God’s plan for marriage and into what marriage is about. It’s not a “no because no” kind of “rules”.
There is also no concrete strong reason to cohabitate before marriage as opposed to waiting those couple of months.
You may also wish to browse the threads in the Family forum, even just titles, and see how many of them are about spouses changing their mind on things, leaving, going on birth control, causing other distressing situations. Refusing to compromise on what our faith teaches about morality doesn’t give a sure-fire guarantee but it can go a long way towards avoiding such problems. That and also honestly appraising the quarrels and tensions and the factors which cause them, especially those associated with differences between you and your prospective spouse.
Also, if a woman flatly says there’s no deal unless you cohabitate before the wedding, then you can flatly tell her it’s no deal then. You can have requirements too. You don’t need to be the one who budges.
Also, sometimes courage is what is required to get out of commonly accepted typical arrangements. Inaertia, pressure, powerlessness, reluctance to stand out are typical inhibitions. You and your girlfriend can break them and, with God’s help, conquer them (again, in so far as your current situation does go against some real morals and not just perceptions), move on to a better life in your lives. It’s worth it.
