Personal unions can be frustrating, to say the least. I worked so hard, and was only 2 provinces away from forming Scandinavia...and I get stuck in a PU under Castille for 150 years.
Unless it was a 150-year long reign of one monarch, why didn't you just spam insults to get out of the PU?
I had a PU under Castille as Scotland in one of my first ever games. It was a mighty confusing affair and my first taste of the Personal Unions mechanics. Fortunately I'd eaten up England before then.