"When you spend nights with [OM], it makes me very sad, and at times, makes me feel taken advantage of.
I'm not asking anything of you, and I'm not looking for a response. I'm simply expressing my feelings.
That's all. I hope you enjoy your weekend."
I'm not 25yrs, but at first glance, I would suggest you not use the above example as a response. Not that I am great with wording anything, but as a WAW, she may see this as sounding weaker than you intend it to be. It isn't really answering her subject of celebrating (if I understood it correctly) and you may choose not to reply at all. I would just not tell her how sad and taken advantage she makes you feel when she's with OM. Also, are you sure you want to say all of that and turn around and tell her you hope she enjoys her weekend? Isn't she spending it with him? Sounds a little wrong to me.
Just my thoughts.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!
