Honestly buddy, this is quite common.
When you play with some one so often, they will begin to read your game. Some players read games better than others. And some improved their game coz they have the drive to do so. In this case, they have the motivation...it's you.
You've been beating them for so long that they begin to improve their game playing against you especially. Another possibility that some players have what we call "falling into a pattern". Predictability in this case is your own worst enemy.
It's time for you to assess your game. Are you "falling into a pattern"? Are your team mates returning your shots more often now? Shots that used to be winners, are now put back into play, which indirectly frustrates you, and made you to try harder, hitting harder, inducing you to make more mistakes, easy mistakes. If this is your problem, then maybe this is your problem.
If you're an established player, it's quite difficult to improve even by 5% without a coach. On the other hand, if you're a mediocre player, it's easy to improve even by 20% without a coach. Most established player needs coaches to lift their game, which otherwise stagnant and heading for a plateau.
On the topic of trying new things, I would say yes. BH push, chop, topspins and sidespins are must have if you want to be a complete TT player. If you're not good with any of these, you have to train and learn them. You have to do them in trainings and in matches. You may say it will contribute to your loses even more, but it will be temporary. Get those shots, and it will add more dimensions to your game. Learn, train and apply..don't quit if don't quite get it, keep at it. More often than not, it's all in your head. Train until these shots are 2nd nature to you, shots that you can do without even thinking.
Another, always have a strategy and stick to them. If you miss easy shots, forget about them quickly and get on with the game. And always play 1 point at a time. Stay loose, and don't think about winning so much. I think this is the main culprit contributing to your loses. Imagine..you used to beat these guys all the time, now they're giving you such a hard time. You lose most deuce games, and when you win, you win by a margin. You put too much pressure on yourself to win that you begin to tighten up. I bet you're saying in your head "I can't believe I am losing this match"...Once you have that in your head, you already lost. When you're leading by a big margin, you're able to close the game easily..coz no pressure there.
Remember, do the simple things, but do them well. Don't worry about those flicks, just because the pros do them. I think it's time for you to step up your game (and mind), or otherwise your opponents are going to have a field day with you.
Learn those BH topspins and pushes, get new serves..all these will increase your confidence by leaps and bounce. Trust me...I've been there 30 years ago