I don't even know...

Me and this guy have had this on and off relationship, but we're never really "off". Just not "dating". But we're still like FWB. when are in our FWB part of our "relationship" I feel kind of used. It makes me feel anxious to know that one day he may be mine (in bed at least) and the next be someone else's. Idk what to do. Is this healthy?