The guilt FEELING is just a TOOL to get us to look at our behavior and make sure we aren't doing anything deliberately that might hurt ourselves or another. You are not responsible for her financial difficulties! She had a space to rent and you rented it in good faith, thinking it would work fine but found out by EXPERIENCE that that wasn't so. It's a learning experience for both of you.

I would apologize for submitting the apartment rental application before you talked to her but I think that is the only thing you are responsible for, using her as a reference without telling her you were doing so. Maybe there is some way you can help her look for a replacement renter? Can you afford to move out a litter slower and pay her an extra week's rent, anything like that? Normally, in a formal rental situation one gives 30 days notice of moving.

But I would try not to feel badly about what cannot now be changed; see if you can think of way(s) to help her in the immediate future and move on. I think action would help both of you and your bad/sad feelings.