A senior position requires more than just technical ability - in fact technical ability may be one of the less important aspects.

Personally, I know that I'm more technically capable than most other developers that I've worked with, but I'm lacking in some of the 'soft' skills areas (e.g. I can clearly and eloquently get my point of view across in a document or via email, but if people ask me in a meeting, I tend to stutter, forget what I was about to say etc.), I don't have any leadership experience and I can lack empathy with people who don't see things from my point of view.

Until you have experience working alongside other people, you won't know what areas you lack in.

I really want to say how much I appreciate the good responses here. I think most of what I have learned for project management has been from collaboration. I frequent local meet-ups.

I do want to point out that you can manage a project with 1 developer, and failing to do so will almost inevitably lead to a snowball of spaghetti that is impossible to maintain. I think that was an important thing to realize for me. I treat my own projects as though I am a team of 1.

Obviously, surrounding yourself with experts really does help. I don't have a particular mentor per say, but talking to a few people with 10-30 years sure does help. They almost always have the most brilliant insight(and yes its usually a lot simpler than what it seems).