Nervous!

I have this problem.
I sit down at a table and I feel like I'm dominated before we even start (playing fixed limit on the net).

It's almost as I expect to lose. Sometimes when I'm doing really well I get more agressive and not bullied so easily. When i'm doing so-so or even bad, I'm scared as soon as someone bets. Not to mention if someone raises my bet.

What can I do to play more secure? I hate being scared most of the time.