My partner’s boyfriend just decided to cut me off and stop talking to me and on top of that, my partner barely communicates with me anymore (even though theyre in constant communication with the bf) despite me letting them know that my depression is getting worse and i’m feeling more alone. Any advice at all on how to approach this??

This sounds like an emergency to me. The first thing you need to do is reach out to someone who isn’t part of this situation for help - ideally, a therapist, but if you don’t have one, a trusted friend or someone you can rely on for support. Depression is not something to mess around with. 

It is possible that your partner’s boyfriend feels overwhelmed or doesn’t know how to handle the fact that he’s very close to someone who’s dealing with mental illness. It’s crappy, and it’s not your fault, but not everyone has the capacity to handle tough situations like this. And it’s possible that your partner also is struggling with how to meet your needs as well as their own. 

My suggestion is, once you’ve established a safe avenue of support outside this poly network, to talk to your partner about what’s going on. Say your needs are not being met and you’re feeling alone, and ask them whether they’re feeling able to be there for you and what they need. You may find out that they can’t be what you need them to be right now, which will be difficult, but you can use that information to start building the support structure that you need rather than asking someone to help you carry something that’s just too heavy for them to hold.

I am sorry that you’re dealing with this. Best of luck. <3