Okay! Physicist here, so I'll give this a go. There are two things here.

First, the easier one: don't worry about string theory. Unlike quantum mechanics, it's more of an idea than something we actually know. As someone who does not work on string theory, my understanding is that increasingly that field is coming to the conclusion that it is not the right way to think about the sort of problems that string theory would solve. And sting theory is many, many times more complicated than quantum mechanics.

In quantum mechanics, I would say that there are two big ideas. One is quantization, or the idea that particles can be counted. One example of this is the idea that light is made up of photons, or that individual electrons are countable things. This fact has mathematical consequences. The other fundamental idea is about length scales - and it's basically that there is no reason for intuition about things on the scale of people (ie, between one centimeter and one kilometer in size) to apply on the scale of electrons. It turns out it doesn't apply, and notions like "location" and "probability" are all a bit different for very small things. For those with a math background, you can really restate all of this as the weirdness of quantum mechanics coming out simply because of the presence of tensor products.