Improving and beating your PBs can be a big challenge once you get past newbie gains, so anything that helps can be useful and distraction is a much under valued tool when it's getting rough.
1) I like to divide down my session into percentages. So when you get to a certain milestone (metrestone?) you can start telling yourself your x% through and when you get to halfway remind yourself that you have done more than you need to do...it's downhill from here on

2) For longer slower sessions I have recently started to count down the metres left. For example with 4k left in a 16k session and I'm hitting a rough patch, I note the metres remaining at the start of that stroke then for every stroke, at the end of the drive when I catch my breath, I say to myself one of the single numbers e.g 3987m, will be split into 3,9,8 and then 7 and then glance again at the screen and you'll be surprised with how much it's dropped. Then start again e.g. it might be 3960m after another four strokes you'll have taken off another chunk. The key to this is distraction and establishing a bite size chunk of progress: your mind is quite easily distracted but it needs to be constant otherwise it will start to focus on the pain.
3) For shorter sharper sessions I found it really useful, especially in my recent 20 x 300m session, to count up to 15 strokes and down then down from 15 to 0. You might need to count to 16 or 17 but all that matters is that when it gets tough you're counting down the strokes, or you could count down the metres, left to the finish.
4) Mantras. Anything short and repetitive will do. I liked 'I Can, I Will, I Must' when I was doing my ultra distances. Again it's all about distraction and this could be a favourite song lyric, film quote or anything that means something to you as long it is short, repetitive and memorable.
5) Focus on the splits and see where you are at a certain point. Splits are useful at splitting down the session so choose them wisely.
6) Imagine your family and/or friends are stood behind you cheering you on. I vividly recall my wife jokingly saying when I left to go to the gym to do my 100km " and don't you dare come back until you finish the 100km". This bounced around my head when i was struggling and close to quitting after the 15th time of stopping and lying on the floor.
7) Pride. Focus on your pride when you achieve your target and your head is swimming in endorphins. Short term pain for long term pride.
There are probably loads of other coping strategies but hopefully these will give some strength to some of you when all you want to do is HD.
Anyone else got any other nuggets to pass on? Never forget, progress happens too slowly to notice and setbacks happen too quickly to ignore. Don't let the setbacks control your mind.