Sure, just turning off notifications doesn't really work - your boss is going to message you anyway, and you'll keep checking to see if anything important came in! That just creates more stress and guilt. What's much more important is actively talking to your boss. Tell him specifically: "After 6 PM I won't be responding, only in emergencies." Most bosses respect that when you make it clear that you'll be fresher and more productive afterward. That's not laziness, that's being professional!
What helped me is: creating a physical boundary. If you have a separate workspace, leave it when work ends! Close the laptop, put your phone in another room, done. And this is really important - fill your free time with something that actually distracts you. Don't just sit around and stay mentally at work. For me it works when I go outside or do something hands-on - then my head is really off. Your girlfriend's right that you're irritable, but you can change that. Give yourself two or three weeks until the new routine sticks.