How can one improve his chess game, beyond the basics?

Lately, I have not been doing well, and I am just asking if you could give me any advice.

"... for those that want to be as good as they can be, they'll have to work hard.

Play opponents who are better than you … . Learn basic endgames. Create a simple opening repertoire (understanding the moves are far more important than memorizing them). Study tactics. And pick up tons of patterns. That’s the drumbeat of success. ..." - IM Jeremy Silman (December 27, 2018)

https://www.chess.com/article/view/little-things-that-help-your-game

https://www.chess.com/article/view/how-to-start-out-in-chess

https://www.chess.com/news/view/a-new-years-resolution-improve-your-chess-with-new-lessons
"... 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
"... 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
Here are some reading possibilities that I often mention:
Simple Attacking Plans by Fred Wilson (2012)
https://web.archive.org/web/20140708090402/http://www.chesscafe.com/text/review874.pdf
http://dev.jeremysilman.com/shop/pc/Simple-Attacking-Plans-77p3731.htm
Logical Chess: Move by Move by Irving Chernev (1957)
https://web.archive.org/web/20140708104437/http://www.chesscafe.com/text/logichess.pdf
The Most Instructive Games of Chess Ever Played by Irving Chernev (1965)
https://chessbookreviews.wordpress.com/tag/most-instructive-games-of-chess-ever-played/
Winning Chess by Irving Chernev and Fred Reinfeld (1948)
https://web.archive.org/web/20140708093415/http://www.chesscafe.com/text/review919.pdf
Back to Basics: Tactics by Dan Heisman (2007)
https://web.archive.org/web/20140708233537/http://www.chesscafe.com/text/review585.pdf
https://www.chess.com/article/view/book-review-back-to-basics-tactics

https://static1.squarespace.com/static/5856bd64ff7c50433c3803db/t/5895fc0ca5790af7895297e4/1486224396755/btbtactics2excerpt.pdf
Discovering Chess Openings by GM John Emms (2006)
https://web.archive.org/web/20140627114655/http://www.chesscafe.com/text/hansen91.pdf
Openings for Amateurs by Pete Tamburro (2014)
http://kenilworthian.blogspot.com/2014/05/review-of-pete-tamburros-openings-for.html
https://chessbookreviews.wordpress.com/tag/openings-for-amateurs/
https://www.mongoosepress.com/catalog/excerpts/openings_amateurs.pdf
Chess Endgames for Kids by Karsten Müller (2015)
https://chessbookreviews.wordpress.com/tag/chess-endgames-for-kids/
http://www.gambitbooks.com/pdfs/Chess_Endgames_for_Kids.pdf
A Guide to Chess Improvement by Dan Heisman (2010)
https://web.archive.org/web/20140708105628/http://www.chesscafe.com/text/review781.pdf
Studying Chess Made Easy by Andrew Soltis (2009)
https://web.archive.org/web/20140708090448/http://www.chesscafe.com/text/review750.pdf
Seirawan stuff:
http://seagaard.dk/review/eng/bo_beginner/ev_winning_chess.asp?KATID=BO&ID=BO-Beginner
http://www.nystar.com/tamarkin/review1.htm
https://web.archive.org/web/20140627132508/http://www.chesscafe.com/text/hansen173.pdf
https://www.chess.com/article/view/book-review-winning-chess-endings
https://web.archive.org/web/20140708092617/http://www.chesscafe.com/text/review560.pdf

Thank you all.

both good ideas.

 I completely agree with everything that was said.  I especially like the part of the quote that was  emphasized.  Focus on developing good habits and gaining quality experience over playing a greater quantity of games.  You will improve more by playing fewer games and working more to improve from those games you play.  Blitz games should only be used as an occasional change of pace in your training.

Watch these videos, will help you tremendously: 

https://m.youtube.com/playlist?list=PLl9uuRYQ-6MCBnhtCk_bTZsD8GxeWP6BV

https://m.youtube.com/playlist?list=PLl9uuRYQ-6MBwqkmwT42l1fI7Z0bYuwwO

The idea that weak players should avoid learning concrete openings don't have the same validity today. With today's  new tools players can invest some time learning a basic repertoire at least against 1.e4 and 1.d4 as black and go on doing tactics. As white players can try something like the Cole or the London System (there is a short book on the London for free in Chessable) . My major problem was how to play against 1.e4. I came to the conclusion that I should not be too ambitious and play the Scandinavian with... Qd8, ...c6, often... Bf5, ...e6, ...Nf6, ...Be7, ...Nbd7, 0-0. There are some aggressive lines when white sacs is bishop on f7 or attack the black bishop on f5 with pawns, but black can deal with this if he learn well those lines. I recommend the book by GM Colovic available in Chessable.