if owning 20% of provinces or below, they don't feel much of a suicide factor.

if 40% or so, AI factions start to feel the itch and your govs act up.

60%+, govs are venal and the AI is rabid.

80%+ gives random rebellions, better to not have a gov due to crazy unhappiness and also plain suicidal AIs.

you should always minimize your borders and never have to really defend more than 6-8 borders which you can do.

As the Brits, you should have an especially nice time as your longbows can mow down tons of attackers.

Even if you lost a bunch of provinces and suffered civil war or whatever, I don't see the game as being lost. keep fighting!