[jyu]

It sounds like you need a programming mentor or peers. Someone that can help you think through the hard parts, and possibly lend some of their experience. If possible, go to programming meetups where you do not just meet other programmers, but also code together. In lieu of peers and mentors, you could also pay someone to do a code review.

Alternatively, if you feel you are stuck, you might be using a suboptimal approach to solving these problems. Usually any complicated program can be broken down into approachable and smaller chunks. You could also write a really crappy, but working version and use http://codereview.stackexchange.com/ to get your code in better shape.

"Intermediate exercises" by themselves probably will not help you as much as other activities. You've already stated that you continuously run into these walls. It might be good to do some analysis on why, and talk with others on what makes these problems so challenging. Working through these challenging parts can be ridiculously insightful and fruitful in clarifying your understanding of something, or fixing bugs in your mental models.

A couple other related thoughts: You could use your side project as a playground to practice new concepts as you learn them. Also you could get a willing and experienced mentor.