It's well known that politicians promise 1 thing and then do the exact opposite. It happens over and over again. This leads many people to say "you just can't trust politicians" and give up.

I think, we can't trust their words, but we can trust their history. If they have consistently voted (in parlament) in a specific way, then we can believe that they will continue to do so. So, they can build trust by doing politics a certain way for some years. This is probably a much more relevant way to look at politicians than to look at what they say.