Leaving the collective alone (other than to control rpm) is just a discipline, try sticking your left thumb out so it touches your thigh, so you have an additional cue to follow that you're pulling early.
Judging the flare is a tough one and I can still cock it up on occasions. The only hint I can suggest is flaring in segments and seeing what happens after each burst. That might help develop your feel better than an all or nothing haul back. You can count them off out loud by saying flare 1, flare 2, flare 3. It also means you can make adjustments between each one, pause a little longer if you've overcooked it, or go straight into the next one if you need to be a bit more aggressive.
Hope that helps.