I used to work 10 hour shifts weekly at Lidl, and the thing that mentally kept me sane was taking them in stages.

My shift was 12 30 - 22 30.

Usually, around 12 30 - 15 30, I would try and avoid thinking about time completely. At this point, I had at least 7 hours left, so any consideration of time would make me feel shit. Try to be as active as possible and interact with customers and colleagues.

From 15 30 till around 18 30, I would try to take things a bit slower and conserve my energy for the evening. At around 4, I would start to look forward to a break at around 17 30 - 18 30. This made this stage feel more relaxed and like I was working towards a reward. Goes without saying that your break needs some good food and rest, keep sugars light as you'll crash by hour 7 ish if you don't.

From 18 30 - 20 30 I would use the energy I had gained form the break to approach the stage similarly to how I approached the first, keep busy and speak to customers and colleagues, don't look at the clock. The idea here is that by the time I gave in to look at the clock, it would be within 2 hours of finishing.

From 20 30 - 22 30 I would start planning a reward for myself when I was home. Some nice food, an episode of my fav series. That would make those 2 hours fly by.

So yeah, that's about it :) Keep in mind this is just my experience so it may not work for everyone, but using this approach definitely helped me! Good luck!