Petra -

Add to your training: practice at 'off' hours first. When you do move onto the roads, do it at times when traffic is lightest. Then gradually work your way into the busier time frames.

The old adage 'just do it' is somewhat appropriate here. I was very nervous the first time I rode in real traffic (as opposed to weekend rides on rural roads) but the more I did it...the more comfortable I became. I used to get serious nervous butterflies every single day leading up to my commute. Every day! Now I don't. I'm still super wary and overly alert out there, but it no longer makes my stomach turn!