I've lost my job confidence - help!?

I was made redundant from my job as an admin assistant after two years, and then took a job as an Executive Assistant for a big consulting firm. My confidence had dropped a lot after the redundancy, and being in the heady corporate environment at the second job didn't help - I didn't feel like my usual bubbly self and became timid and withdrawn in the workplace. Feedback from colleagues was that I was very quiet - not like me at all. So after 9 months I quit and got a job in advertising - something I had wanted to do for years!! I am working in a very small office with just my boss and his son, and I love the work, and they are both lovely to work with. The thing is, my confidence is still so low that I constantly question my ability. The girl before me was let go, from what I understand, yet the work she left seemed to be perfectly fine. With no handover, I don't really know how I should be approaching the role, and I spend every day feeling paranoid and anxious. Considering how long I've wanted a job in advertising, I should be coming home every day feeling happy, but I just feel scared that I will be let go. What can I do?