My biggest advice to you would be to keep at it. Merc energy management is challenging at all levels, but the passive buffs in the skill tree make it easier as you level. It's normal to struggle in the beginning, especially until you have Supercharged Gas (28) for regular heat dispersion. Until then, make sure you're specced into the Improved Vents utility and practice avoiding overhealing. You've got bursty heals, so use em appropriately. Rather than repeatedly cast Rapid or Healing Scan, use kolto shot to top off.
Again, it's simply going to be difficult until more of the tree unlocks. But stick with it, as it's a very dynamic and rewarding class in the endgame.
If you've played Commando healing, it's completely the same. Ammo works identically, just reversed on a 100-0 scale instead of a 0-100 scale.