My partner agreed to open our relationship, but has been acting distressingly over it

im always open about wanting a poly relationship, but recently I realized I cant do mono forever & told my partner. They were a little hurt, but ultimately said they wanted to open the relationship. Since then though, they've broken down crying about it twice (haven't even been with anyone else yet). I dont feel right about this, but they keep insisting its better than losing me. They refused counseling with me, & idk what else to do. Theyre REALLY going through a lot rn, but idk if i can stay

Don’t stay in this relationship. Someone “going through a lot” doesn’t obligate you to stay in a relationship. If you’re at the point of “I don’t know if I can stay” and “they refused counseling with me,” it’s time to get out. When you said you needed to change the terms of the relationship, they may have said that they will abide by those new terms, but their behavior shows that they are not. You don’t need my permission to leave this relationship, but you have my support and encouragement.