I wouldn't recommend healing classes if you have a slow reaction time as they are dependent on you reacting to different situations.

As for DPS, as others have suggested, Red Mage is super easy to pick up and it's a pretty cool class. Black Mage is also pretty cool with potential for tons of damage, and given its slow cast times, it may be a good fit for you if you like it. In all honesty though, just play the class that you like the best. It's a video game and is meant to be enjoyed, so just pick what you like playing and whatever you have the most fun with. If you're not doing the most damage, so what?