First.. You don't need to be learning to putt on Championship green speed. I tried that too a couple of months ago when I first started.. Tried to play in the US Open qualifier about 4 times, got frustrated and withdrew every time. Having said that, I played the SAME course a few days ago in the US Open Weekly and shot a 66.. That's nothing to brag about, but just saying, it'll get easier.
Play on Fast and Very Fast greens to start with, those are the easiest to learn speed and break on. Pay attention to the tips in those putting tutorials about how much length to add or take off your putt to adjust to the green speed. The speed you put on your putts is critical.