As someone with ADHD, I can tell you what that feeling of losing it is: you're running out of (human brain) working memory. In your case, that's because you're still learning this, and so you're holding the complexity of not completely understanding it in your working memory while trying to pile other things in there, and you hit your capacity for complexity.
The approach I take when I hit that wall is to look for something, anything, that I can do that will make one part seem simpler. Then, when you get a couple of those things nailed down, your brain will have more room to work and everything will start to seem simpler as you proceed.
Writing down a list of things you want to accomplish can sometimes be enough for this phase if you're not too stuck.
Another thing that can really help when you're looking at multiple data sources: diagrams, showing where every source of data is and how it flows through your app. Called a Data Flow Diagram. The best book I've found for how to do them is an oldie, but still worth it, and used copies are cheap (from $3!) on Amazon last I looked: Tom Demarco's Structured Analysis and System Specification.
I still do my diagrams on paper, because I think the act of handwriting helps my brain out, but it may not work that way for yours.
The good news is that these steps can make large and complex apps possible, you just have to know when you're hitting the wall and how to fix that.
(For those who don't know, many of the symptoms of ADHD are caused by less working memory.)
