The more interviews you do, the easier it will get and you will probably become more confident and fluent. Telling whoever's interviewing you isn't a bad thing, either. I told my interviewers that I stuttered. It's not easy, so I can definitely sympathise. I remember when I was applying for jobs and there were interviews that went well and I had almost solid fluency and others where I completely bombed and couldn't get any words out. I went for dozens of interviews, kept applying and eventually got hired. You're getting to the interviewing stage so that's good. Don't give up!