What exactly is your plan for changing first past the post?
You could make the case that if the democrats actually supported that, it's worth holding your nose and voting for them in order to open up other options in the future. But they don't support it, because they benefit from it. So basically you're asking the left to keep voting for the democrats unconditionally forever while they don't address any of our concerns and refuse to make any sort of reforms that might allow us to have a voice in the future. How is that a viable path to accomplishing anything?