Founding a company doesn't entitle you to lifetime tenure. Like every employee, you have to earn and re-earn your standing - and companies are pretty unsentimental about that stuff. If you stop being useful, you get quickly shuttled to the side. That said, the Founder does have a distinct advantage that comes with the title, and with that a unique opportunity to contribute. It takes self-awareness, and the confidence to hand over the reins, but it can be a very healthy and productive long term role.
You are going to go through periods where you feel like you've made a big mistake. As your friends progress along more established paths, you'll often feel like you're falling behind. Get comfortable with it. It's the life you've chosen. And when you do go through those times, just remind yourself that you won't be the one, fifty years from now, telling your grandkids you wished you tried starting a company.