Should a man feel bad if the woman who earns more pays for things?
typically had more money. But those old rules, and that economic situation, were based in entrenched sexism. The problem you’re having is due to the entrenched sexism that lingers in all our brains. You “feel bad” about something that is actually really, really good: you have a generous girlfriend who wants to share her success with you. As for the “imbalance” in your relationship, there’s always been an imbalance between men’s and women’s access to money and power. The only reason you’re noticing it now is because you’re on the other side of the fulcrum.
As long as your girlfriend doesn’t abuse her power as the higher earner by thinking she gets to make all the decisions she’s (mostly) paying for — whether it’s choosing a restaurant or a luxury destination for your next holiday — you could have a very good life together.
Forget the old rules. Be nice and open and honest with each other, and go for a relationship of equals.