That same clever bloke once gave me some advice on castings, and i will share that nugget with you now,

when you get a casting, that means you are already good enough to do the job.      simple isn’t it?

Just knowing that, boosted my confidence more than the next guy and I believe its really helped me. The casting director isn’t going to bring ANYONE into that room if they dont think you can do it, otherwise they are going to look pretty stupid.  So, you CAN do the job! you just need to get it.