My 'end game' used to be about the same as yours. I'd get nice table runs and then myself out of position for the 8...very frustrating.
In 8 ball, the more of your opponent's balls that are pocketed, the better. Leaves you room to work...providing he/she doesn't runout on you.
3 key points to a successful outcome in 8 ball, IMO, is: cue ball control (this includes speed, not just placement), pattern play (and being able to 'adjust' your pattern for changing table lie), and shot-safety, shot-safety, shot-safety.
Shot-safety is not the same as playing a safety. I will often take a more difficult shot just because of where the cue ball will come to rest, as well as your object ball, should you not pocket it. It takes alot of forethought at first, but will quickly become 2nd nature, as will pattern-play. The trick here is to minimize available balls for your opponent to shoot at at any turn at the table.
I now use the pattern-play concept to 'read' my opponent's table lie, so that I can keep him tied up and out of areas of the table that will allow him/her to get out.
I am not very good @ explaining this; I do it much better.

I can not recommend enough a book called 'The Eight Ball Bible' by R Givens. It will teach you all about pattern-play and shot-safety. The diagrams are designed around a 7ft barbox. I diregarded the diagrams, and just concentrated on the theory. My game exploded! Worth every penny!!
Lisa