You don’t
You have lived with diffuse boundaries for some time and are now reaping the penalties. You can and should proceed with care and grace as you implement boundaries and define roles to move to where you want to be but it is absolutely foolish to think that it will not be at least a little hurtful to your pseudo partner.
they will implement their boundaries in response to changes you are proposing; you have to respect these. If they chose to leave and tell you to fuck off then you have learned a valuable lesson in why you shouldn’t let boundaries be so diffuse for so long through so many changing contexts.
It’s not realistic (usually) to expect you to know everything you need and want from a relationship up front but when contexts change you need to clarify what is and is not okay. If you’re okay with keeping it casual after things don’t work out that’s fine but make sure they’re aware. If they suddenly have to move in consider the boundaries of the situation again: are they still cool with keeping it casual? Are they now that you share a bed? Are they now that you’ve purchased a bed together?
If you’re the one that wants it casual and wants the door open for new relationships it’s your responsibility to make sure your partner is aware of where you stand. One could say your friend/partner is foolish for assuming you’ve changed where you stand, and they’d have a valid point, but one could also say that you’ve been very misleading here. Boundaries need to be enforced and they need to be occasionally reviewed as contexts change, otherwise they fade away
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