I have been searching all over the web for answers and still not able to stop the leak, this is last my last resort before I have to get a plumber, So here it goes.

I have a continuous bathtub leak coming from the spout, It is a single Handle, the brand name is MIXET. Instead of replacing washers and o-rings, I decided to replace the whole stem. I followed the instructions and thought I did everything correctly, When I turned the water back on it still was leaking. Just to make sure I didn't forget anything, I took the whole thing apart a second time and did it again. And it still continues leaking. I really could use some help in this matter since this seems to be something as simple of replacing a part somewhere. What am I forgetting? If possible, please answer with diagrams or pictures.

Did you use a wire fitting brush to clean debris from inside where the stem goes? Did you flush it out before you reinstalled it? Did you use a clone or genuine parts?

I did clean the debris and I did flushed it out. I notice that there something else in the copper pipe. I'm guessing this what is being called the "seat' from a previous answer in this site. There is like a rubber washer around it. What I have read about this seat is that if the opening is hexagonal, there is a special tool to take this out. If it is a round circle it cannot be taken out. Well if this is the part that sounds like it need to be replaced how do we take it out because it is a round circle? Now I did go to a hardware store and they told me that there is a tool like the one for the hexagonal opening but I would need to hammer it into the opening and then pull it out. I don't like the idea of hammering anything into copper pipes. What is your opinion about this?

Actually I have not worked on a mixet faucet. I know how to get a rounded seat from a typical faucet but if this is different, you would mess up the whole faucet body