When she says
“I’ll let you know” = She has other plans of higher priority (girlfriends, new guy, etc) You’re the back up plan.
What you do = You make other plans & tell her “sorry I’m busy tonight” (if she pops up and says “what time?”
“I’ll let you know” = She has other plans of higher priority (girlfriends, new guy, etc) You’re the back up plan.
What you do = You make other plans & tell her “sorry I’m busy tonight” (if she pops up and says “what time?”

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