Struggling in a primary normally means you'll struggle in the general.
This is simply false. All republicans would suck up and vote for Jeb Bush when it came down to it. He's perfectly conservative enough, has excellent fiscal achievement in Florida and has a heart that you can't quite find out of the other 20+ candidates in this race on both sides. I don't care what people tell polling companies right now, the Republican base would come down to election week with Hillary Clinton as the Dem nominee and would get off their ass and vote for Jeb. He's not George Pataki. Jeb is absolutely conservative on various issues and conservatively moderate on a few issues at absolute worse.
Jeb is playing for the general and seeings as how the GOP base would suck up and vote for him, then he just has to broaden the base outside that, which is what he looks to be doing.
Jeb struggling in the primary is not a sign he'll struggle in the general. He's struggling in the primary because he is playing for the general instead of catering to the GOP base this early on.