First off, how to not care so much - go through something like this! Rethink & analyze your position and reasons and feelings. Decide to "just say no" on some things that you realize you have only been doing out of obligation. And you give others the same option to do so. And you shrug your shoulders and say "oh well, it's their choice, nothing I can do about it" and move on. Don't dwell on it.
Now something else this is making me think about- no offense to you but hear me out.
I had a roommate/best friend in college who would get upset over little things, she seriously was melodramatic over stuff that were the usual daily disappointments - I actually remember her saying to me that she was gonna go throw herself in front of the bus one month and something else like jumping off the cliff or a building the next month. It puzzled me because she was a girl who "had it all" she was popular, athletic, never hurt for dates, was close to her families, etc. The reasons for her drastic woe is me attitude that came for about a few days each month were honestly trivial. A lower than expected grade, a lousy workout, a date couldn't make it, etc.
She was also very regular and one month at the end of her period, she went into my room and lifted the calendar page and marked a day off. I was puzzled, and she said that was when next month's was coming. Huh ok. (here I sit, jealous because I'm lucky if I have them 2 months in a row! I never know when mine is coming!) Anyway fast forward to next month. Whoo, she was in tears and upset over something that, like I said, was a disappointment but NOT the end of the world. I glanced at the calendar and, aha, it was a few days before her mark. I pointed that out to her and realized she had PMS (which she hadn't thought was a real thing. She and I think that it is overused as an excuse or license for bad behavior sometimes!). After that, she was able to be a little more aware and realize that her extreme feelings were caused by hormones, not that the triggering causes were truly big deals. It was hard but a very good realization, helped her to have a little perspective.
Now, this may not be the case AT ALL with you, but if this feeling is coming out of the blue, and the situation is totally horrible and unacceptable, when it wasn't such a big deal a week ago, or isn't a week later, then PMS or hormones could be amplifying the situation here.
Cindy, I am not saying that I wouldn't have my feelings hurt either, that I wouldn't be wondering how to not care so much either, that this is a big deal over nothing because it's the time of your month. I'm just suggesting you consider whether the timing has anything to do with it (and making it worse). I agree, so very often, we take things personally, we try hard to be nice, then of course we are hurt when we find others aren't being as careful to be as nice back to us, etc. That is life. That is part of the evolution we all take, and hopefully most don't swing from one extreme to the other extreme, but instead learn to steer a little closer to center. I mean this very sincerely and kindly.