When do you eat your dessert? Before you eat your meal or after? What do enjoy more: first have free time and then work or first work and then have free time? What is more pleasant: walking downhill and then uphill or first uphill and then downhill?
I’ll take a wild guess: you have your dessert after your meal, you enjoy your free time after work and you find it more pleasant to walk downhill after having walked uphill. My guess is not exactly wild because this is simply how most people approach things that resemble a task & reward situation.
Interestingly, this is not how most people approach their language learning sessions. Actually, many people try to avoid uncomfortable tasks altogether and instead work on and repeat what they are already good at. This is especially true for vocabulary sessions, as people seem to enjoy the breezing through decks of words and phrases getting the sweet feeling of confirmation that they indeed know their stuff right in the beginning of their study session.
Unfortunately, this leads to a bit of frustration once they get to the words they don’t know and on top of it, their energy level at the end of the session will be lower. After a while of studying, you might get tired or hungry or thirsty. In other words, if you save the hard work for last, you will be in a worse state mentally than in the beginning of your session, which will make the tough part of your work even tougher, and as a result, you will feel less satisfied in the end or even demotivated.
The easy solution is, of course, to do the exercises that you feel are more difficult for you at the very beginning of your session. That way, you can spend a good part of your energy on the tough task. Finish your session with something you find less challenging, for example going through vocabulary that you are already fairly comfortable with, or a listening exercise you have done before for reenforcement. Doing that will make you close the session on a positive, reassuring note. You will also have covered more than you would have if you had had your dessert first.