There are layers to the responsibility that we all have for the games that we play, and lose. To name some of the prominent ones:
- Was the deck I chose the best deck I could have played at a high level today?
- Was the maindeck optimized for the field I projected?
- Did I call the field correctly?
- Was my sideboard optimized?
- Did I keep hands I shouldn't have kept?
- Did I play lines that had a lower percentage chance of winning?
- Was I successful at bluffing my opponents, and leading them down paths more likely to lead to my victory?
- Did I give information away with my reactions in playing?
- Did I test appropriately for the event?
Each of these points could be expounded on at length, but I'm a bit crushed for time at the moment, so I'll just let them sit as is.
Finally, when you test, do you test with players who are better than you are? Do you know what specifically you're looking for when you play? You need to realize what turns the match (sometimes it's blatantly obvious, sometimes it's not), and then see how you can change that.
Regardless, good luck. Being a better player is difficult, and it takes time. We were all terrible at one point, and it took many of us years to truly get better. I'm currently playing mediocre Magic in Dominaria drafts, so I feel your pain, as I keep reminding myself to RTFC.