At a general level, the key to becoming a better player is to focus on the little things and understand whether the decisions you made were correct or not given the information you had available to you at the time. The posters before me made a lot of great points but I wanted to add a few things that didn't jump out to me from what has already been written here on the topic.
I have extracted a lot of meaningful insight by taking copious notes during serious testing sessions and tournaments so I can revisit key sequences after the fact. I write down all of my opening hands and spend a few minutes after most matches to document any specific details which might be useful down the road. I recommend adopting a similar approach and really committing to revisit your games to understand what happened.
In addition to taking notes, I encourage you to seek out players who are better than you and befriend them. This was the single most important thing that I did as a Magic player and I am supremely confident that I would not be anywhere close to the player or person that I am today without @Prospero as a mentor. Not everyone is as lucky as I was, but finding a good sparring partner is invaluable. Whether you actually play games together or just bounce ideas off of one another, it is significantly harder to succeed as a lone wolf rather than as a pack.
Finally and possibly most importantly, make sure you are having fun. As @Brass-Man said, getting better is a job after a certain point and it doesn't pay well, so make sure you are enjoying yourself. Personally, I try my absolute hardest to playtest in person with friends that I can play fun, competitive games with and only play MTGO as a last resort. If or when it stops being fun, don't hesitate to take a break. Personally, I play sparingly between November and February of each year to give myself time to unwind and recharge before diving into my preparation for the 6 or so paper events I play each year. Without this annual sabbatical I would have burned out long ago.