I need a big help about my martial arts?

You should absolutely still train Boxing. I believe in complimentary training. That meaning training in different styles and techniques that cover the weaknesses of others. What you've got going on is a good example of that. Your Karate gives you a foundation in kicking, and punching techniques (Im only guessing as much because I dont know exactly what style of Karate you do). Boxing gives perfection in punching techniques as well as better movement, stances and blocking. Its true its gonna mess up your karate technique, but its going to improve your fighting technique. Now you have the movement, blocking and effective stance of boxing as well as the kicking ability of Karate. Really what you've got going on is the foundation for Mixed Martial Arts. You should keep training and maybe even think about transitioning into kickboxing, as well as start working with grappling techniques. You';re on the way to becoming a well rounded "Mixed" Martial Artist