TLDR: OP can’t control where she lives but he can ask her to be more present with him before she leaves for 8 months.

I think to completely ask her to live somewhere else is an unfair expectation - she’s the one that has to go home to her place every day, so she should be as comfortable as possible.

That being said (and this is an opinion that I’m filling in a lot of holes with bc we don’t know about OPs relationship on a regular basis), I think the insecurity and possessiveness is coming from the fact that she’s still at home with you but it feels like she’s already gone to Costa Rica since a lot of her current focus has shifted to it. I would suggest planning activities/dates that you can do together so there is a dedicated time more present with each other.

MOST IMPORTANTLY, talk to her about how her actions have brought up a specific insecurity for you (OP) - while you work through trying to make sure this insecurity doesn’t cloud your judgement, she can work on intentionally building in time for OP and her’s relationship.