How to change header menu heirarchy

How do I get my horizontal menu bar under my header to only drop down some of the subpages? At the moment one of the headings has about 50+ subpages and it’s messing up the layout when you hover over it.

Also, I’ve just finally managed to sort out my sidebar only showing subpages to the selected parent page, but there’s a small glitch in that it only works when I keep a link to a completely different page in there, which looks a little odd. It’s not a huge problem, just slightly annoying. If there’s a solution, I’d love to fix it.