My partner and I have been together for over a year and poly for just as long. I have major depression with psychotic features and I think my now excessive worry that he’ll prefer another women over me after sex, stems from that. The anxiety has only increased as of late bc of some relationship stressors (Temporary LDR, cheating) I’m wondering if it would be fair to close our relationship while I start treatment to lessen stress or if it’d be better to keep it open and work through my fears?

There is a lot going on here. First off, good on you for starting treatment! I think figuring out what is healthy for you with regards to this situation might be a good early issue to work out with the mental health professional you begin seeing.

Either way, I would bring this issue up with your partner - say you’re thinking of closing the relationship temporarily, and see how he responds. He might agree with you and suggest a closure, he might not realize this has been such a contributor to your anxiety, he might be curious to see what your doctor says, or he might be against the idea (which would be a red flag about his ability to support you through this). He should be involved in your treatment and recovery, and he should be aware of how certain relationship stressors are affecting you so you two can work towards a healthier situation together.