I don’t think it’s fair to say “All millennials are like this because blah blah technology blah blah modern world!” You’re hedging right now, placing the blame of your failed relationship on culture.
Being happy with what you’ve got is a difficult art, but I have one possible fix: don’t get involved with anyone unless they’re That Great. After my first failed relationship and the string of disappointing dates that followed, that’s what I resolved to do. Fast forward two years later and I’m in a happy, solid relationship with the person I will probably marry. Learn how to be alone and you won’t feel the need to latch onto every relationship that comes into the realm of possibility.