I've felt a bit like this. Have you got any holiday left? If you're able to, take a long weekend, give yourself permission not to do work.
I needed the break - so spent so much time fart-arsing around, but tired at the end of day because I had still forced myself to sit in front of the laptop.
I also had the call of the rest of the house, so somehow feeling guilty I'm working when the dishwasher still needs doing etc.
I also have a little extension on chrome, called Otto, that can block some websites, and follows the Pomodoro method, working 25 mins, 5 min break. While 25 mins doesn't seem like very long, you'll get a fair bit done, and will help at least get back in the habit of focusing for a while, so you can build up to longer. You might find, because 25 minutes isn't an intimidating amount of time, once you get started you find yourself working much longer.
Can you rearrange your workspace, I find sometimes that helps. Just feels like I'm sitting somewhere different, and it feels a bit fresher and easier to focus.
Are you able to talk to your manager, and are you behind in tasks? While it's probably "right" to make up the work and time, it doesn't actually help - you don't get a proper break and you're overall less productive - still feeling guilty. The number of days I've started early in the morning, but then waste the extra time (and more) in the afternoon. Spend some of your work day making a plan to catch up - mix a combination of "new" work, and "backlog" work - starting with the most urgent.
If you can talk to your manager, and be honest with how you're feeling - and come with even a little plan about how you're trying to get back on track, it will hopefully ease the guilt and they can help with your workload and make it easier to face.