OK. Flat and high contrast is a contradiction in terms. A negative can't be both at the same time.
To me it seems that the negatives are 'dense'. That could have happened for two reasons:
1. The film is fogged somehow.
2. You gave the film too much exposure.
It seems that on top of that you also developed the film longer than recommended, so I'm guessing it's a combination of overexposure and over-development.
Before you start working with a different developer, do yourself a favor. Sacrifice a roll, and bracket exposures of static object in average contrast lighting. Shoot it at 125, 160, 200, 250, 320, and 400. Two different scenes. Develop normal, say D76 1+1 for 11 minutes. Evaluate your negatives by looking at shadow detail only. Print the two you think have the best shadow density, using your normal paper, and at the contrast grade you usually like to print, and make your decision which effective film speed works best for you, without looking at the rest of the tones for now. Evaluate only shadows.
Next shoot a roll at the film speed that gave you the best shadows. Evaluate the frame with the best shadows from the first roll, but now look at the mid-tones and the highlights. If the highlights are not bright enough, this next test roll should be developed longer. If the highlights are blocked up, the roll should be developed shorter time. Adjust until you have a negative that prints well at your normal print grade.
You can get to this point with any developer, and giving up after using only one roll is not a good way to progress to good results. The mechanism is to make your negatives print well on your paper. In the beginning that can be like putting square pegs in round holes, but after you adjust your negative tonality, by first evaluating film speed, and then evaluating needed developing time, you will have a good recipe for making good negatives moving forward. This is the underlying principle for even having different developing times, to adjust to your individual needs in combination with making sure that all pieces of the puzzle fit in nicely.
