I started my first business about 25 years ago. I was the least likely person you can imagine, very introverted and academic/bookish. My motivator was that I was a single parent with no money and the dc need winter coats/shoes (and it kind of grew from there into something huge). At the time, I had one friend who was very successful in business and he was so 'ordinary'- just like me, if that makes sense - I just kept telling myself that if he could do it, I could. And like you, I had a lot of experience and already knew I was an 'expert' in my field. (Remind yourself of that often!)
I think part of it is also finding someone who inspires you, who can give you the extra boost when you need it. I really like Tony Robbins, for example (not everyone's favourite, I know). When I went through a really tough patch in the business, I'd just read Richard Branson's book, 'Screw it. Let's Do It'. At the time I was having to step out of my comfort zone almost every day. I got through it by thinking, 'Ok, what would Richard Branson do in this situation?' It sounds a bit daft, but it really worked.
There have been some very tough times (e.g. following the 2008 crash and then my exh, who was also a director, walking out just as we were getting back on an even keel) and it can be difficult to pick yourself up and go on afterwards. I had to completely start again in 2013. (Interestingly, it never occurred to me that I shouldn't do so.) It helps to take the position that even if the 'worst' happens and the business fails, so what? You've learned something and you'll do it better the next time. It's the business that has failed, not you.
It will also help if you can find a way of detaching yourself from other people's opinions, because they are not always as supportive as you might hope. Sometimes the people closest to you are the worst, and the more successful you are the worse they can be. I've had to ignore a lot of people over the years.
In other words, I think it's about learning what/who motivates and inspires you. Also becoming comfortable with the idea of 'failure' and 'setbacks'. Ask yourself, 'what's the worst that can happen' - usually, it's not that bad. And once you have faced it, you can put together a plan for what you would do if that did happen.