It is not a problem that markets changed dramatically.
The real question is: did you adapt to these dramatical changes? Changes can also have a positive impact. It is not by definition a negative one.
If you ccould adapt (or even profit from it), then there is no problem. I trade since the 90's basically the same system without any problems. The last 5 years the performance even increased very much.
A good system adapts itself to the changing conditions. The challenge is to find how to create that self adapting part.
In the S&P you clearly see differences, one of them is this:
in 1995 S&P was around 500; 0.1% rise would be 0.5 points or $25 per contract
in 2021 S&P is around 4,200; 0.1% rise would be 4.2 points or $$210 per contract
So today percentagewise the same move brings 8.4 times more profit. Margins went of course up too, but the limitation of tradable size is moved much higher. To make the same profit in $ in 1995 as in 2021, you need to trade a size 8.4 times bigger than today.