Why you want to know that do you have mild or severe stammering. A person who stammers is a stammerer and one should accept it. Now the main thing is to how you can manage your stammering and be good at communication. You should rather focus on practising your speaking skills with lot of people. Every stranger is an opportunity for us to improve our stammering and be confident. Talking about um word, in technical terms it is called helping word by which you speak your main word. My suggestion is to prevent taking help from helping words and try to speak the actual word which you wanted to say initially. If you will keep on using helping word, it will soon become habit and then it would be much difficult to get rid of it.