Yup, been there, done that, got the t-shirt.
Grounded Spirit is right - this is actually pretty normal stuff for poly. GS and LR have some solid advice, which pretty much jives with my own experience, too. Talk to everyone involved, be loving & respectful, but don't be afraid to set some boundaries if you have to. It's OK to have your space and your stuff be sacrosanct. Nothing wrong with that. Don't beat yourself up.
We've found that it works better if we don't think of it as relationship "work", which can be overwhelming. Think of it as something fun instead, so that you are motivated to do it rather than intimidated by it.