Sometimes when people have a mood disorder they will act out especially when feeling depressed. People find ways to convince themselves that they feel better by not taking their medication and then looking for ways to help themselves superficially through such behaviors as sexually or emotionally looking for comfort. It is possible that she has reasons for not taking medication like worrying about side effects or even the inconvenience and then seeking comfort by impulsive sexual behavior. One has to totally accept the idea that medication provides long term care rather than convincing themselves that they do not need the medication. This is very common with psychiatric medication where one will feel that they don't need the medication to feel better but then seek out less effective ways to be stable. The other possibility is that the relationship is going through a time where it is in trouble but she refuses to look at why. Both could also be working together which results in her irritability.
I would let her know what your expectations are and how you are suffering with her behavior to see if she is willing to put in the work. You could also recommend that both of you meet with her psychiatrist to voice your concerns. In addition there are sites for online therapy that you could access to work on this as a couple. Depression does bring irritability and that is usually managed through medication.
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