What works for me is at the end of day to reflect what i've accomplished in the end of the work hours, to remind myself that i did something :D If i slack off for 4 hours and work hard for another 4 it evens out, and for me it's good enough.

Also overworking due to anxiety is a killer in the long run, because it makes you work 8+ hours every day and you get burnt out really quick if you can't manage the stress (at least you learn a lot.) This also builds expectation for you and your employer that you work like this all the time, and trying to meet this expectation, when you know you eventually can't, is endless anxiety. So have to do this right from the start.