There could be many possibilities for what is going wrong. What are you studying? What sort of games are you playing?
"... In order to maximize the benefits of [theory and practice], these two should be approached in a balanced manner. ... Play as many slow games (60 5 or preferably slower) as possible, ... The other side of improvement is theory. ... This can be reading books, taking lessons, watching videos, doing problems on software, etc. ..." - NM Dan Heisman (2002)
https://web.archive.org/web/20140627084053/http://www.chesscafe.com/text/heisman19.pdf
"... The game might be divided into three parts, i.e.:- 1. The opening. 2. The middle-game. 3. The end-game. There is one thing you must strive for, to be equally efficient in the three parts. Whether you are a strong or a weak player, you should try to be of equal strength in the three parts. ..." - Capablanca
"... Just because a book contains lots of information that you don’t know, it doesn’t necessarily mean that it will be extremely helpful in making you better at this point in your chess development. ..." - Dan Heisman (2001)
https://web.archive.org/web/20140626180930/http://www.chesscafe.com/text/heisman06.pdf
"... The books that are most highly thought of are not necessarily the most useful. Go with those that you find to be readable. ..." - GM Nigel Davies (2010)
"... If it’s instruction, you look for an author that addresses players at your level (buying something that’s too advanced won’t help you at all). This means that a classic book that is revered by many people might not be useful for you. ..." - IM Jeremy Silman (2015)
https://www.chess.com/article/view/the-best-chess-books-ever
"... [annotated games are] infinitely more useful than bare game scores. However, annotated games vary widely in quality. Some are excellent study material. Others are poor. But the most numerous fall into a third category - good-but-wrong-for-you. ... You want games with annotations that answer the questions that baffle you the most. ..." - GM Andrew Soltis (2010)
I study chess every single day, and I play many games, both online and offline. However, I still play really badly. I miss obvious tactics all of the time, I play too slowly, get into time trouble constantly, and everyone plays faster than me. I'm slowly getting more and more frustrated. Chess is stressing me out and I don't know what to do. It's not fun anymore. I just can't break out of this mess I've gotten myself into, and it seems as if I can't get any better. I'm putting in tons of effort to get nothing out of it.