Please don't become one of those healers that let tanks drop to 10% because they are DPSing, being constantly in the edge of death is bad, specially for the tank's heart. As with everything in life, balance is the key.

I recommend that you follow this priority list:

1. Safety over DPS, always. If you are unsure about the incoming damage from a mechanic or a pull, stop DPSing and top off everyone just in case.

2. The priority list for healing is: Healer>Tank>DPS. If tank dies, you can raise him and he'll keep aggro even if some DPS dies in the process. If you die, it's most likely a wipe, so prioritize urself over everyone else.

3. If you know the incoming damage and you can handle it, do DPS, but never people drop below 50-70%, because if they fail a mechanic or step on an AoE they are going to die. If unsure about incoming damage, check point 1.

Optimization of DPS and heals comes with experience, so just keep playing, watch guides, ask better players and limit test yourself.

Everyone who plays healer has missclicked and wiped the party at some point in their career, that's fine as long as it doesn't happen often, after that you have to learn how to avoid getting to the point where 1 missed heal makes you wipe.