I think your problem is that you don't see the coordinate system in the good way.
While you go down the Y value is increasing! Not decreasing like you see in most math classes..
To fix your main.lua you should just replace those:
jumpHeight= -200 --in love.load
player.y_velocity = player.y_velocity - gravity * dt --in the love.update
if (player.y > 303) then --in love.update
I hope this is clear for you, good luck for your game (I like the graphics).
Also, if you want help faster you should directly post to Love2d Forums.