Total Pressure = Static Pressure + Dynamic Pressure.
We need to be careful in using the statement that "Total pressure is constant".
It is more correct to say that: "Total pressure is constant at all points in a stream tube provided no energy is added to or subtracted from the airstream."
To understand this let's look at an aircraft sitting on the runway in still air at ISA mean sea level. Static pressure is approximately 15 PSI and because the air is still, the dynamic pressure is zero. So the total pressure is approximately 15 PSI.
If we now accelerate the aircraft, the dynamic pressure will increase with the square of the TAS. Eventually a speed will be reached at which the dynamic pressure is 15 PSI. But this does not mean that the static pressure has fallen to zero. The increased dynamic pressure was caused by the extra energy that we provided by accelerating the aircraft. So the ambient static pressure will still be approximately 15 PSI and the total pressure will be approximately 30 PSI.
If we continue to accelerate the dynamic pressure will become greater than 15 PSI, so the total pressure must be greater than its initial value of approximately 15 PSI.
At the stagnation point the airflow is brought to rest and this converts all of its dynamic pressure into static pressure. The total static pressure (the stagnation pressure) at the stagnation point is then the sum of ambient static pressure plus the dynamic pressure that has been converted. In the case of our aircraft flying at a speed at which dynamic pressure is 15 PSI we would have 30 PSI of stagnation pressure, but only 15 PSI of ambient static pressure.
Later on in your studies you will look at the effects of shock waves. When air flows through a shock wave it is abruptly compressed. This converts some of the pressure energy into heat. This reduces the total pressure. So as air flows through a shock wave the total pressure of the air stream decreases.