I don't think that is exactly true.

For one, given that we are not perfectly rational, then we are certainly capable of holding inconsistent or even contradictory beliefs, provided that we aren't aware of the contradiction in the positions that we hold. For instance, if I were a religious die-hard Trump supporter, I may believe that committing adultery is wrong when asked generally in accordance with the bible, but in the case of Trump's affair with Stormy Daniels claim that it isn't morally wrong for one to cheat on their wife.

In addition, the issue also depends on what one takes "belief" to mean. Some people may believe that there can be unconscious beliefs that one may hold on top of the conscious ones. If that were the case, then that would also mean that one can "believe" in not-p yet still "believe" in p. I may claim that racism is wrong and may genuinely believe in my mind that all people are equal, but also be unknowingly biased against certain ethnic groups regardless.

That said though, I don't think your statement is entirely wrong. If one consciously and actively denies p (or affirms not-p), then I don't see how they can also consciously and actively affirm p at the same time. Perhaps this is what you meant by your thread title. It may be possible for one to hold contradictory beliefs at different times or in different ways, as I have just described, but I can't conceive of a mental state like the one above.

I agree but not entirely. You can have degrees in the strength of your beleif or disbelief. That can depend on the person

I belive that is not true or i don't beleive that is true can have equal weighting in the mind of the person saying either of these things further evidence can strengthen or weaken their beleif or disbelief