>>33784881 (OP)
Well, you can't, obviously. What you need to do is trick your brain into thinking it's not such a big deal. Don't ever think "I need to go to the gym regularly." Instead think "Maybe I'll go today. Maybe I won't even use any of the machines, maybe I'll just surreptitiously check out some /fit women. Better dress for it, so I don't get busted...". Then, when you're actually there, think "Well, I've come all this way now, haven't I? Seems dumb not to at least do a few bench presses." And then think "Well, that wasn't so bad, maybe I'll try some others." But never, in your head, commit to going more than once.
Rather than going for a run, think of it as as a chance to listen to some audio books in peace.
If you have a huge project to do, never let yourself think about the whole project. Instead think "Well, I could work for 25 minutes, couldn't I? Then I'll take a break. Maybe I'll do a little more after that, but probably not." Do 25 minutes. Stop. Get up, walk around, stretch, etc. for 5 minutes. Then think "You know what, maybe I could actually do another 25 minutes. Definitely not committing to more than that, though." Do another 25; stop for 5. Then think "Eh, that actually wasn't actually quite as bad as I was expecting, maybe I could manage another 25?" And so on - don't ever commit to doing any more than 25 minutes.
>inb4 Pomodoro
Yes, but that's not the point.
For a presentation, don't think of it as a presentation, think of it as a stage performance: do it in character, don't be you while you're doing it.
You can find plenty of other ways of fooling yourself if you try.