Seth and Eric gave good answers, and Shweska is right about grammar nerds, but I think even they won't find fault with this, because it is simply different ways of expressing something :) May I ask which dictionaries you checked?

I'd say it can go either way. "The pressure of life" is like.... all the combined pressures. Everything, altogether. We use the singular form more often, and it's what we use when we talk about pressures in general. "I've been under a lot of pressure lately." "All this pressure is really getting me down." "Being the eldest son of a wealthy family sure is a lot of pressure." But I might say "pressures" when I want to emphasize that there are a lot of different responsibilities weighing down on me. This is not a common usage, but it definitely exists. "The pressures of being a teenager are too numerous to count." "The pressures of social media and constant connection make it difficult to relax even in your spare time." "I can no longer bear the pressures of living in society, so I'm going to run off to the woods and become a hermit." In short: normally uncountable, but you might say "pressures" to emphasize that you're under a lot of different types of pressure.

It is both depending on usage. Pressure as a characteristic of a gas or an element like water is uncountable. So it is used as a singular word. “The gas chamber has a lot of pressure” for example. But figuratively, pressure is used to describe stress or strain like the pressure of taking an exam or the pressure of a job interview. Together, these are some of life’s pressures, as there are more than one activity that has pressure. Here, pressure is countable.

Hi Grace! "All the pressures of life" is acceptable in regular everyday English. Nobody could tell you your usage is wrong if you used this phrase. Its another matter if you are surrounded by grammar nerds.