how to make clear I [f22] am serious and not just with him for the older experience?

many times when I met older guys that I liked and could really imagine a relationship or in general something deeper I feel like the guys thought I just wanted to have a little adventure because of their age or was dating them for the money. And no matter what I told them,they never believe me and now I want to know what can I say or do to make clear that I’m serious about this? :(