I'm about to give you a bunch of advice I don't follow myself. Hopefully if you follow it you'll get better results than I do
You just need to play a LOT. That's why heavy MTGO players get results, it lets you play all the time. If you have a local regular group that's willing to test multiple times a week, that's probably better than MTGO, but realistically you don't (nobody does, that I know of).
When testing, look for ways to extract more data out of your games. Being objective is super important. Take notes if you have to (it's super easy to lie to yourself about results if you're not recording them). Every test game should have a specific purpose. If you're building a deck or studying a matchup you should be trying to answer a specific question. (e.g. what are the most important cards to coutner/remove, which turn one plays correlate to wins most consistently). If you're testing in paper, there are ways you can answer questions more efficiently by not strictly following the rules while testing ... seeding cards into your opening hand, skipping games where one player mulligans too much. Just remember that staying objective is hard work.
If you're trying to generally improve playskill rather than improve a deck, you can also test with specific goals in mind. Try things like "this match, I'm going to concentrate on playing my lands and tapping mana perfectly," and don't worry so much about mistakes elsewhere. Identify your weaknesses and concentrate on them one at a time. In theory you could record games on MTGO and re-watch them with a friend later, and fully talk through any scenarios where you weren't sure you made the right decision. This is likely very powerful, but I don't know of anyone who goes this far in testing.
You're probably not playing enough post-board games. Nobody is, really. I'm not sure it makes much sense to play pre-board games at all, except under extreme circumstances, after huge amounts of other work has been exhausted.
Of course, add all of this up and you basically have a job that you have to pay money to work, so make sure whatever your testing strategy is, you're enjoying it. You'll burn out real fast if you don't, and it's tough to beat anyone after you ragequit.