It's a really fun game with simple but clever mechanics. It's also cool to use Greek mythology for the setting. Who doesn't enjoy getting flung to the sky with a boulder. The physics of the boulder is very well done and conveys its weight.
I think that the gravity scale of the player is a little too low. Not sure if it's intended but it is possible for the player to pull the boulder instead of pushing. As you have noticed, having instant kill when there's no checkpoint is a little too punishing.
Great job making the game!