As a Paladin, in even Tank Spec. I slaughter. I'm almost always at the top of the charts.
You should have many more spells than that. Perhaps you pulled a Pen and didn't upgrade any of your spells? xP Hehe.
At your level, those 2 action bars should be full. I'm shocked it isn't, honestly.
*EDIT*
Above is a picture of my damage done as a Tank at level 40, and my action bars. Some spells are on there in certain places just... because.
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Also, Pureown is correct. Exorcism should not be used so often. I mainly use it as a tank to help pull groups while currently tanking, or just to finish undead off. There are many more useful spells that you should be using.
you pretty much answered your own question. You shouldn't be spamming that, your mana will just drop. You better get a new weapon haha. All you really have to do is cast it once, judgment, and then normally attack. what is the dps on the 2h?
As for your attacks you should have them in this order of priority.
Hammer of wraith(when targets at 20%) - Exorcism(Only during art of war proc) - Judgement - if you have 3 holy power use Templars Virdict - crusader strike(single target) otherwise devine storm.
Looking at the picture it also looks like you're not using a seal. I don't really know much about paladins seals, but I'm pretty sure one of them restores mana.
Also, when you use your judgement spells you regain a LOT of mana.