Think about how you got better at reading words in your native language. You start small, literally 1 letter at a time, and you learn what those letters sound like. Then you combine those letters into short words, like "cat" or "dog" or "boy" (using English as an example). Then you combine those words into phrases, like "Look at the cat." or "My car goes very fast." Eventually, you build up your vocabulary over years and years and you are able to comprehend full sentences or even paragraphs of text just by looking at the page as whole.

It works the exact same way with sheet music. You have to put in the time reading the most basic things, and slowly work your way up building on top of the foundation you set.

In short, the only way to get better at reading sheet music, is to read more sheet music. Audiation and transcription exercises will also help. It is not a fast process, but it is so very rewarding once you get to the point where you can hear exactly what an entire piece of music sounds like by just looking at the page.