Adult USTA league tennis is a form of competitive tennis that is, in my experience, distinct from other competitive tennis situations, especially those encountered at the high school, juniors, and even college levels. I played tennis from middle school through high school, competed in juniors tournaments, took loads of private lessons, etc. I didn’t play in college because I was burnt out and decided to quit tennis after I graduated high school, but I was probably equivalent to a D3 or low D2 player. When I was in grad school 8 years later, though, I started playing again for fun and, eventually, registered in a USTA league.
When I first started USTA, I self-rated at 3.5 and, despite being a “textbook” player with much better technique than the majority of my opponents, I didn’t win a SINGLE match my first two seasons. Like you, I felt so disheartened, especially because I had played my whole life and I felt like I “should be winning” because “my technique is better and I hit harder.” Well, it took me two full seasons to shake that (arrogant lol) attitude and come to accept and respect the fact that USTA is an environment in which you will necessarily encounter a much more diverse range of players and playing styles than what you would playing juniors, college, etc. This is because you are often playing against self-taught players, players who picked tennis up later in life, players who have mastered “bad technique,” players who have been in USTA leagues for 20+ years, etc. I didn’t start winning until I a) got out of the somewhat snobby mindset that better technique = more likely to win, and 2) got a bit more practice under my belt playing different types of players than I would have ever encountered in the past when I was in juniors (pushers, slicers, lobbers, etc.) I think you literally just need match practice against a lot of people, and this includes becoming more familiar and comfortable with the format of USTA matches and the overall “vibe” of the league. Some leagues/regions are a lot more formal and than others, and I think this can sometimes cause a lot of nerves for those who are new to adult league match play, especially if you’re generally younger than the majority of your opponents (which was my situation). Hang in there—I promise you’ll start getting better results with time!
I’m now a 4.5 and I’ve been playing USTA for about 10 years, and I can tell you that even at 4.5, players who just came off of D2 or D3 college teams a few years ago who self-rate at 4.5 still lose matches to “weaker” players a lot in their first season playing USTA because they often are just not prepared for the huge variety of play style, pace, and technique they end up playing against. It’s just a totally different world of tennis competition!