Leaving longtime company

Hello

Been at my current company over 15 years but have been very unhappy with the way the company operates. After a long search finally I found a new job.

Current company is quite large and new company is much smaller. Also I’ve become very negative in the last year due to before mentioned unhappiness either company direction.

I’m a little nervous about starting over and I’m just wondering how other people deal with the feeling of uncertainty and having to prove themselves again after so long.

Thanks in advance.