Mac, things would be a bit different except that you keep seeming to miss the main point:

She's a barista. Her job is to be very friendly. That is literally what she is paid to do. The chance of her wanting a relationship or a friendship from someone that she sees regularly at work is very slim. The relationship that you feel there is... it isn't genuine. Is there a chance it could be? Sure, it does happen. But it's very slim.

The further the age difference the slimmer it will be, because most people (any gender, age, dating-preferences) don't tend to form relationships (friendships or dating) with people outside of a relative age-range to themselves.

Should it leave you bitter? No. You misread a situation. That happens. It's ok that it happens. Feeling disappointment is totally natural.... but bitter? She's doing her job, and hasn't done anything wrong at all. You just happen to be in a minority for dating preferences.

You'll be more likely to have positive results if you find ways to connect with women closer in your age range.