For reasons I don't want to get into right now I ended up playing as part of a team that competes at a level I can't match. I have lost all 15 of my singles matches, up to this point.
I have never played in a league before so I didn't have "real" match experience up until the start of this season, apart from one tournament I played (and won easily, at the lowest level). I've done 1.5yrs of training once/week as a teenager and recently started playing again after about a 15 year break. My opponents all have multiple decades worth of matchplay experience.
The odds are stacked against me to say the least, so I can't fault myself for losing, it's just really annoying. Only the best players take easy wins against me, so you'd think I would at least be able to win a few matches here and there.
I came close a few times, but no cigar. Sometimes I start out strong (e.g. an 11-3 first set win against one of the top players in our league

by pressuring him non stop) but they learn and adapt and then simply out play me on every level

. I can't keep up my "in the zone" very best play for long enough. Today I started one match being outclassed by a very very very strong looper/hitter. (He hits so hard I couldn't even get my racket to the ball in time to block most of them, whereas normally against people at or slightly above my level I'm particularly strong in open play counter topspin rallies.) I got better and better against him by playing short angled shots, but still lost in 3 sets, the last by 8-11 or 9-11. My doubles matches aren't quite as bad, winning over half of them.
Even if I come close, they seem to cope much better with the pressure, especially ending a set, even more so if it's the last set. My shots are not consistent enough. Sometimes they just play a bit passively and let me make the mistakes, and it works. However, changing tactics to a more pushing, passive/opportunistic game I have tried to do but the longer a point goes on, the lower my chances because I'm much more likely to make the first mistake than they are. So playing a safe game doesn't seem to work either. It seems I have to pressure them, play the angles, and be "in the zone" enough to be hitting the ball very well, in order to win. Problem is, I can't keep that up long enough to win a whole match. When it gets to 2-2 and the fifth deciding set is played, they always take it. (They go up a notch and I get tense and therefore down two notches.)
All I want at this point, is to win one match against one of the weaker players.
I won't be able to make all my shots more consistent overnight, so I guess if there is anything I can do about this before the season is over it'll be something psychological (tension is definitely one issue) and/or training specific things needed for matchplay that people don't normally tend to train for.
Any clues?
Edit:
Summary of what's been suggested in this thread thus far:
Train serve return and serve with my coach (largely dictates your level).
Find a partner to do drills with.
Lock in on your opponent's elbow and keep hitting it there. NEED TO DO THIS MORE
Train block, push, flip/flick.
Nothing to lose. Let go and you could play better. CHECK (doesn't seem to produce results)
Take notes on HOW and WHY you lost a match. CHECK
Stay aggressive, unafraid to attack toward end of set/match (without being reckless). CHECK
Focus, focus, focus. Get book "the mindset" to deal with tension. CHECK
Actually finish reading it.
Set longer term goals and forget about losses in the interim. CHECK
Play lots of real matches against stronger players than you. CHECK
Consider all of your losses as part of your training. CHECK
Stop flat hitting into the net, add topspin.
Change your game and tactics after you've won a set, so the opponent doesn't know what is coming next. This may mean trying to keep some of your weapons off display in the first set. The second thing to think about is finding a different way to attack the weaknesses you must have exploited in winning the first set.
Make them believe you are the better player even if you doubt that.
Follow up psychological momentum with your trickiest serve, to help crush your opponent's spirit.
Look for your opponent's mistakes and punish them.