Heart Leveling is a simple mod to improve your health in the game by spending xp on it. Improve your health with the help of earned xp in the heart leveling menu, which opens with the "H" button (can be changed in the control settings).
A lot of hearts are not so easy to feed, so you can always lower your health up to 10 hearts:
You can also change the required amount of xp for each heart in the mod config.