If you tell it to the judge just like you wrote it, He will probably dismiss it, but you'll probably end up waiting for another court date...I think thats why people just pay the fine and put it past them. Then you will probably have higher insurance as well, so I would fight it.
As I understand your question, you think you were OK to merge because you did not realize how fast the other car was moving up on you. I believe you were ticketed fairly. It does not matter that the other driver was speeding or drunk, although he does bear some responsibility for hitting you, of course. It is still your job to stay out of his way, if possible. This is why we are taught in Driver's Ed not to rely only on our mirrors, but to actually turn our heads to check for oncoming traffic. The mirror will not help you at all if the oncoming car is in your car's "blind spot", for instance.