Tanking Question and Help

Ok, so I healed in most big MMOs for years on end, and I ended up rolling a BLM but I didnt enjoy it that much, so now im working on a GLD/PLD my question is around the rotation, Right now its like this...

I open with a shield lob, then flash, then FB,SB,RoH,SB,Flash,SB,SB, and repeat. most of the time its fine, 1on1 with a boss is cake, 2 mobs is usually fine, but sometimes when I have 3 and 4, it gets trickey, my main question is, how do you guys handle 3+ mobs, I usually shield lob then flash, attack the main one, flash again and tab and hit each other one once with a SB then back to the main, but sometimes the healer will grab aggro on the 3rd or 4th mob, what could I do to improve this? thanks.