For me, to overcome that isolation, I found that I had to engage people in the industry more, engage with activities and people that I feel were reasons I started. This helped me fight the isolation. Another good side affect of doing this is that I was able to meet more customers, engage them directly and use that to help me not feel so isolated.
I also feel finding someone you can turn to and just vent that understands you are not fatalistic but have to vent so you can get it out of your head to refocus is critical. Sometimes you get 2 people, one too vent to like that, and one who can mentor you in the business aspects. I am lucky that I had a mentor that would/could do both, and he has the credibility on business.
Today I have more than one mentor but for different things. I am also not a founder currently but it is only a matter of time before I start something new. And given my past, there is a high likelihood I will be solo again unless I just ran into the right co-founder.
Option B: Find your #1 fan. Just tell random people about what you're doing. Someone out there will be a fanatic who gets excited about every update. Some might even propose things or offer resources to help. Sometimes you can promote them to a cofounder (mine started at 0% then 5% then 15% equity). It might seem like a bit, but the moral support can be surprisingly powerful.
Option C: Promote a hardcore employee. At some point you'll get employees who really believe in the vision, sometimes even more so than the founder. This seems to be the second most popular support pillar for solo founders.