I often wonder if I too easily approximate ~80 years as infinity in my reasoning about life

Imagine a large paper divided into 100 x 100 squares. One of those squares, most likely at the bottom part of the paper, contains an invisible mark. Every day you remove a square. When you remove the one with the invisible mark, you die.

(If you are a student, you have more expected days remaining, but you should be aware that they will be on average less useful than the ones you have now, because generally, as you grow older, you have gradually more duties and worse health. So you probably should imagine 100 x 100 days, even if you have more, to compensate for this bias.)

Too much or too little? Think about the activities and plans you were doing recently, to calibrate yourself how many squares would each of them cost. (Include the days when you were too tired or too busy to do anything useful, because those days passed too.) This gives you an idea about how much can you do. Now think about the things you did 10 years ago; how happy you are you did them; how many of them do you even remember. This gives you an idea about how you will feel 20 or 30 years later about how you spend your time now.