I'm currently a solo founder. Beyond the normal challenges, three areas have really hurt me. First, without the perspective of a co-founder, I haven't noticed several issues that really should have been obvious much sooner. Second, without the accountability of a co-founder, it's been hard to devote as much time and energy to the important tasks that I'm not good at as to the tasks I am. Finally, without the support of a co-founder, it's hard to find the time to get ahead of problems and out of firefighting mode.
The most valuable thing I've done to combat these is to find good advisers, meet with them regularly, and really dig into their advice. However, these things have been difficult to overcome by myself, and I'm currently looking for a co-founder.