One thing about arcane is that you don't really have a rotation. What you have is a throttle control.
I like to equate arcane to a fighter plane. You can choose to cruise along slowly (AB+ABarr) and fly for a long, long time until you run out of fuel. Or, you can choose to hit full afterburners (AB*n->MBAM) and fly like a rocket, though you'll run out of fuel very quickly. Naturally, you can set that throttle everywhere in between as well. That's the beauty of the spec really, that you can modulate your DPS and DPM GCD by GCD, ramping up when mana is in surplus (evo/gem CDs are up) and throttling back when mana is in shortage. If you stick with the spec, in time this throttling behavior becomes second nature.
I don't claim to be an arcane expert, but here are some tips I've worked out.
If you don't have your mana level visible in a conspicuous place, fix that. You need to develop the ability to forecast your mana usage on the fly. If you find yourself casting at full throttle, at 30% mana with no cooldowns up and the boss still at 60%, you've already lost. You develop that skill by watching your mana carefully as you cast.
Don't be afraid of your mana gem. Remember, if you get through a fight without having used that gem every time it was off CD, you didn't do as much damage as you could have. Evocate whenever you can. You have to know fights very well and plan in accordance. Get in the habit of saying to yourself, "Evo is up in 45 seconds. The boss will teleport in 60 seconds giving me a window. I have 9k mana, so I'm going to drop to AB*2->ArBarr/MBAM so I can last." After a while, you won't have to think it out, you'll glance at your cooldowns and mana and throttle back instinctively. Summoning a gem mid-fight isn't necessarily a bad thing either. You spend 2375 mana and <3 seconds for a guaranteed average 3400 return later on.
It was mentioned before, but clip your evocations on IV, Heroism/Bloodlust whenever you possibly can. The couple of seconds you save are a whole GCD, which means more damage. It also means less time spent unable to move, and in the case of IV, a nigh-uninterpretable evocate to boot.
If you're going to play arcane, gear for arcane. Crit is for Fire/FFB/Frost. Intellect is our bread and haste is our butter. A big mana pool means more flexibility, a larger margin of error, and beefier evos. Haste boosts your DPS like crazy. Around 500 haste rating, AB falls under 2 seconds. Add it haste trinkets, IV, and hero/bl. Do you know what it's like to hit 4 12k+ crits in a row, one every 1.3 seconds? ;) Spirit is delicious, delicious honey that boosts mana regen and pumps up molten armor.
So... keep with it. It takes a while to get used to. I wanded a lot of bosses my first week being arcane
, but once I got the hang of it, I knew I'd never go back to FFB.