Another woman under his roof is disconcerting, and I can see how it would cause uncertainty in your relationship. If the romantic part of their relationship is over (and it sounds like that’s long gone), then I think that, if you are willing to hang in there a minute, this situation just might right itself.
We all have varying amounts of baggage. And his comes in the form of a cantankerous ex-girlfriend. Too large to slide under the seat or stuff in the overhead, that’s for sure. And the lid isn’t all the way closed on this one yet, either.
When we date someone new, we often forget to look at the whole package. Mostly we just focus on him and his potential. I know women who forgot to consider all sorts of complicated details, from minor children living with him to ex-wives to aging parents to a mentally unwell brother. We are never just committing to one person when we enter into a relationship; we’re potentially committing to their obligations, as well. The worst of it is we’re committing to entangle ourselves in their obligations while we don’t really get a vote—just ask any stepmother how fun that can be!
So, back to you, love. What are you going to do about it?
This is his hot mess, not yours. And since we’re only talking about storing this excess baggage a couple of months, I recommend you leave it with him to handle and stow away—you keep that part of his life separate. That means you don’t go to his house while things are still being worked out between him and his ex, and you two don’t spend a lot of time talking about the situation. It doesn’t have to be a weird, never-to-be-spoken-about-again scenario, but it doesn’t have to take up much of your time, either. And whatever the final ruling is on the money? That’s not yours to comment on. These are (bad) decisions that were made before you ever walked through his door. So don’t worry about weighing in on the outcome.
I’m not sure how your fear of abandonment fits into this story. Do you think he’s going to let her keep living with him forever because it would just be easier? I’m guessing that’s not his plan. It’s natural to be fearful that a relationship isn’t going to turn out the way you want it to when it’s new. The uncertainty we all feel at the start of a new relationship is one of the things that makes it hot and exciting. So, try and bend that agitating energy more towards excitement and less towards worry. After all, that’s about the only control you have over this situation.