How to tell my close friend that she hurts my feelings and I feel like a backup friend?

I’ve been friends with Monica for 7 years. And she’s one of my closest friends. Her, I and another girl - Emily are like a trio, but Emily now lives in another city.
The thing is I’m starting to feel like a backup friend to Monica. I’m always the one initiating our hangouts, she almost never calls or checks up on me unless she needs something.
This NYE she had invited only Emily (I found out through Instagram) to celebrate and in the morning they both called me and Monica was giggling and said “how was the new year? Emily’s here too” which made me feel very unwanted like she was mocking me. Emily later told me it was Monica’s idea to call.
She also didn’t invite me to her birthday, but did Emily and then kept making plans with her that I wasn’t included in right in front of me. And this has happened several times. I know I’m not entitled to her inviting me, but it still hurts. When the three of us hang out, I almost feel like a third wheel, although Emily tries to include me.
She comes from a lower income family, so each time we hang out, I’m the one who always pays for her food, drinks, movie tickets etc. and I always invite her to my house or family vacations, which is why another friend of mine thinks she’s just using me.
It’s honestly hard for me to imagine her as such, cuz she’s always very nice and sweet, and we usually have great time together when is the two of us, but maybe I’m just too naïve.
Either way, I want to tell her how I feel about our friendship, where should I start and how should I approach the subject? A call, irl?
How to tell my close friend that she hurts my feelings and I feel like a backup friend?
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