Part of the issue is day-to-day variance. Sometimes you feel a little off, be it from sickness, tiredness, not in the mood to play, or something else entirely. Other times you are just in the zone and play far above average. Be aware of these things that cause variance, but don't use them as an excuse; learn to power through them.
Part of the issue is the skill level of your opponents. This should be obvious. Sometimes you do poorly because you face a team above your skill level and vice-versa. This can't be helped too much. You can get consistent scrim partners, however.
Part of the issue might be snowballing self-criticism. Don't be too hard on your own failures during the same game; it's probably gonna make you do worse. Worry about how to improve at halftime and after the game.