You may think of yourself as a fantastic, world class speaker, or you may think of yourself as the worst speaker there is- either way, you are absolutely right!
I will be upfront with you: i frankly think that the motivational speakers talking about positive attitude all the time are full of baloney. this concept is so over simplified that it becomes meaningless to people. I can have a great attitude while going out to golf, thinking of myself being better than Tiger Woods at his peak, yet i would shoot the ball into the woods or the pond. My great attitude does not have an effect on my golf score or skills.
However, speaking is different. I have found that having a good attitude to public speaking, acting like you are having fun at it rubs off on the audience. If you keep acting like you are not good at speaking, then it will show in your eyes and that is exactly what your audience will think- that you aren’t good. If you act like you are good at it, then a part of it will be persuasive enough to the listeners.
Do not act to the point where your overconfidence becomes off putting but in general acting like you are good does pay off. If you know you are good, it will solve a number of your real speaking problems. For example, you will not feel the need to have a lectern or you will not spend the speech staring into your notes.
So along with good practice, you must also tell yourself that you are a good speaker. Only then can you actually become one.