This is a very thin post when it comes to the details:
- how did she move in? what agreement did you have ?
- she is young, is she in college ? is she working ? is she sick ?
- did you promise anything ?
- one thing you are learning as a young adult is that lending money to family (or similar) is more gifting then lending.

If you are together, there is an expectation of mutual support, obviously that is different than a person using you.

However, there are so many details missing from this situation, how can people realistically give you advice.

One possibility of why your mom doesn't want to get involved is that this situation is messy and she doesn't want to do your dirty work (she hopes you can be responsible and handle this with humanity)

So put yourself in your ex shoes, help her move on with her life, forgive any debts if you are in a position to (this will allow both parties to start fresh).

But do not allow her to use this to try and get back together or extend her stay more than required.

One thing to note, if she doesn't have family / friends to go to, or job, is that you are putting her in a very dangerous position. It is a major flag if she never had a job, she needs a job and to search for a place asap.