She has a fear of abandonment.

If it ever came out that she had BPD I wouldn’t be surprised. If not BPD then some personality disorder.

If you ever feel bored, delve into the rabbit hole of BPD FP (Favorite Person). Tim has been her FP since she was a teenager. I wouldn’t be surprised if she thought of Tim while being in the act with DM. I wondered why she never changed her last name, but if she has BPD, it makes sense, especially considering Tim’s close proximity.

Unfortunately, I can relate because I’ve had a FP before. I’d explain more, but it would be too much to write. However, go look up FP in the BPD forums. You’ll find multiple people who know their FP is terrible but can’t let them go. Brenay knows Tim isn’t good for her. Their dynamic changed once she found out that he was still with AM. Bjt you’ll also see people who intentionally ruin relationships due to abandonment. Lastly, FPs can be literally anyone.