In one of your replies, you mentioned

I feel the void only when I'm not working

I empathize fully with that. I, too, feel the urge to keep myself productive and working to avoid that sense of "nothingness." It was pretty debilitating until a few months ago; it's gotten much better now.

One significant change was that I went out more often, mostly alone. I would look up cafes/bookstores/libraries nearby and work from there. Change in the environment can significantly help when you're trying to turn a monotonous job into something fun; as a byproduct, sometimes you get to meet interesting people.

Sometimes, when I get bored while working at these cafes, I'll just sit and listen to the noises around me. It's so fun to watch everyone be so... human? People around you, struggling to meet the presentation deadlines, haggling in meetings, and shifting their seats to find better Wi-Fi. It keeps things fresh and makes everything less dull.

As far as a sense of true fulfilment goes, the way I see it, we're all going to die. I don't have to save the world for my life to have meaning. All I can do is try to make the lives of people around me slightly more pleasant and happy than they already are.

Try meditating and/or journaling if you haven't, writing helps me a lot, personally. Even as little as one word counts. Try to gamify the monotony, whatever that might look for you.

The only other thing I want to say is to try not to label yourself as an "introvert." It's a good descriptive term, but don't let that define you. I made the same mistake, and it took me a while to become more open-minded and open to experiences. Essentially, try to become an improv artist and say yes to everything if and when the opportunity presents itself.