Ancalagon
Dusty Dragon
Milestones done wrong can feel like arbitrary brakes to keep you where the DM wants you. Not a fun thing
It should be noted - the GM (possibly in consultation with the players, possibly not) determines the pace of leveling. If the GM wants a slow advancement, it's their right - they are doing most of the work after all, and know things you do not (like possible plans for the campaign).
If you don't like it, talk with the GM. If that doesn't fix things, you are perfectly free to find another game.