Swim upstream

To swim upstream is to go against the general direction, and so to find resistance to your ideas or actions. This expression comes from rivers and fish: salmon swim against the current (upstream) in order to reproduce. It seems like it would difficult for fish to swim like this, but they do it! So we use this expression to talk about going against the general way, belief, or trend. Here are a few examples:

Although I tried to convince my colleagues that washing their hands would help prevent colds and flu, I realized that I was swimming upstream.

After many years of swimming upstream in the company, Joe finally decided to start his onw business.

I think if you're willing to swim upstream for a while, you can convince people eventually.