You said yourself exactly what you need to do: Test your blood sugar. Who cares that you haven't for months. Start testing more often TODAY.
It's not that your insulin isn't working...it's just that when your blood sugar gets super high and you have ketones it's a lot harder to bring it back down. It'll take a lot more insulin than you'd normally give.
I can tell you exactly what an ER will do for you...I work in one...and I see how our doctors treat high blood sugar/DKA all the time. Trust me, it's not fancy. And it's a waste of money for anyone who's conscious and knows how to treat diabetes.
IF you do go to the ER, this is exactly what will happen: They'll check a finger stick BG, start an IV, draw blood and run labs, give you a bolus of insulin (humalog or novolog), give you liter(s) of saline through your IV, and then possibly put you on a insulin drip of old school regular insulin. Plus everyone's fave part of the E.R. - hours of waiting.
And they'll charge an arm and a leg for it. And sadly it's stuff that you or I could all do at home: check your sugar, take some insulin, make sure you've taken your basal insulin (or your pump is working), an drink lots of water.
Go to your 24 hr pharmacy and buy a box of ketostix. They're easy to use. Take a strip out, pee on it, compare it to the colors on the box and see if you have ketones. If you do, drinks TONS OF WATER. And keep taking insulin until your blood sugar comes down. There's no fancy trick. You're not doing anything wrong. You just have to have the gumption to do it.
I know you can. Anyone with type 1 diabetes has a certain sense of "can do!" that enables us to take control of our health. We might get lazy at times, but it's never too late to fix your bad habits. Get to it, girlfriend.
If you do everything we've been telling you then we just saved you tons of money on a wasted hospital trip. :)