Every tank goes through the "nerves" until they are more comfortable with tanking. It's part of learning to tank
To answer your concerns though - basically watch your healer's mana. If it is above 70%, pull. If less, ask him if he needs mana.
That should answer most of your questions
If you are taking too much damage, the healer will be runing low on mana a lot.
If you are pulling too fast, your healer will be running low on mana.
It's all about watching your healer's mana levels
Of course, this assumes that you are mataining aggro on all mobs you are fighting, interrupting, using CDs properly, taunting when needed, etc...