My parents lived in London in the 60's. There were signs in windows of boarding houses saying "No blacks, no dogs, no Irish". Obviously, it wasn't racism as Irish and British people are both white but it was institutionalised discrimination by those in power.
I lived in London in the 80's. Those were the days when you could be beaten up and/or verbally abused for having an Irish accent. I was never beaten up but I was verbally abused simply for having an Irish accent.
Nationalists in Northern Ireland endured sustained and institutionalised discrimination from the unionists from partition. Catholics were denied jobs and housing. They are also subject to violence. Electoral boundaries were gerrymandered to ensure a unionist majority in Parliament even in areas with a nationalist majority population. The civil rights movement started in the late 60's and continued for decades, although it became a bloody struggle.
Travellers are discriminated against in Ireland and the UK. They are most definitely not in a position of power.
Jews also endure discrimination.
I think that your statement that white people cannot be the victims of racism is not true, if you define race as it is defined in the equality act. For the purposes of the Act `race´ includes colour, nationality and ethnic or national origins. The examples I have given are racism under the equality act. I suppose the advantage white people in discriminated groups have is they mainly look the same as those in power (although travellers and Roma can usually be identified physically) so if they stay quiet they are safer.
I don't think it's helpful to dismiss other's experience as it just creates division. Black people have endured way more than white people. The majority of white people have never, and will never, experience racism as they are, as you say, in a position of power. You won't change the mind of those who dismiss racism no matter what you do so you're wasting your time trying.
What is happening in the US at the moment is appalling. Racism is embedded in the system in many states. Trump actively encourages it with his comments.