Advice Please

Advice Please I have had a on and off stammer for a few years, sometimes it can be gone for 5 months and sometimes it will stay for the same amount of time (I'm 16). It's not really a stammer it's just I can't get my words out so I change the word into something else. I am going for work experience in a few weeks and I am very nervous about it. I'm also trying to get a job but I am extremely anxious. When I am at school and I get picked on I will sometimes give the wrong answer if I can't say what I want to say. I know it's a on and off thing but can anyone give my any advice to get over this forever or am I stuck with it for the rest of my life?. (I try practicing to speak to myself in a mirror and it sometimes helps a little)