of course they can believe what they want to believe - it does not mean it's healthy

I don't see how that should matter to you unless you're under any sort of assault from such people. There are plenty of people who believe themselves to be one gender trapped in the other's body. Unusual? Certainly. Unhealthy? Arguably. A source of problem for those around? Not unless they project actively their vision of themselves onto others.

I had troubles associating with humanity and humans because, until the age of 19, I was never accepted as a person. I had very different ideas from my peers - the idea of mutual respect, of kindness and helping etc. - and as such, I had big troubles connecting to others. You know who I thought I was? A Martian, as a species not terran yet a prominent humanoid. What helped me overcome that is someone acceptance, someone showing me that I'm actually as human as I can be, and all my troubles are of a human being. Was it healthy for me to believe otherwise? Arguably. Did it do me good to be accepted despite my beliefs? Most certainly.

The way I see it, you aren't helping anyone with what you do. If you're coming up to people you barely know and tell them that they have mental issues, who's benefitting from that? No one. I wouldn't listen to a stranger telling me that I have mental issues, especially if their sole motivation is to "get me healthy". "Go fuck yourself, dude!".

People believe whatever makes them comfortable with the world around them, most of the times. It is when brain chemistry messes with the people, then they might need help. Not yours, but those who care about said people and have their trust. You're just putting sticks into others' wheels. Don't do that, or don't be surprised when others do the same to you.