Originally Posted by SchrodingersCat
You were supporting him financially on the assumption that he would eventually use your support to get a higher paying job and contribute more to the household finances. You basically made an investment in him.
IIRC, the OP mentioned that he helped put her through school, so if it ever came down to who "invested" in the other, it would probably be a wash.
But all that aside - it's shitty when we find that something we thought was one way turns out to be a very different way. Here you are (talking to the OP now), cruising along, "practicing" polyamorous relationship(s), dealing with your issues, owning your own baggage, supporting your partner(s) through thick and thin... and the one you live with, whom you've known the longest, whom you consider your family & closest buddy and believe is the one person who has your back no matter what - pulls the rug out from under you like this. I am SURE that a year or two or five from now you'll say to yourself, "What was I thinking? How could I have allowed myself to become in that situation?" and you'll realize you're much better off. You sound like you're starting down that road already. But like others have said, there will be some serious short-term suckage; hopefully you come out of it a better person (not that there's anything wrong with you now).