Find out the factor that is turning your positive stresses into negative stresses. For eg when we have time in hand the stress to meet a deadline helps us perform better but if there is shortage of time then the same stress is negative. So here the factor is time.
Write down the thoughts that come to your mind when you think about public speaking. What could be the worst case scenario? That you would forget and excuse yourself? How would that impact you? Would that impact have a lasting effect?
You can try speaking in front of mirror and your friends to gradually get used to speaking. It also depends a lot about how much knowledge you have about the subject that you are going to speak about. Instead of thinking that you are giving a speech, think that you are sharing your views with people. For interview, have a thorough knowledge about the company or organization. Do not think about the consequences instead take interviews as a learning experience. Gradually your confidence will increase. Before interviews or public speaking try deep breathing and meditate for a while to help you calm down.