Yeah the UI was not great, it was a last minute hack job. There are a lot of information that is not conveyed properly. Mana is basically a time limit on your life, you will die if that runs out. the way to refresh it is by gaining combos, it is there as a way to make you speed up and think fast. I did say not to spam the dash mostly because your dash has a cool down and you don't want to be locked in a troubled location, especially on later difficulties where there are a lot of bullets.
Anyway, thanks for playing the game. I hope you appreciate the effort I did to make the game feel satisfying. :)