Can someone ring on your behalf OP? Is physically going there an option?
Ok, tomorrow first thing, Once calling or at desk say is 'hello, i am quite overdue my smear and id like to book in please' write that on a piece of paper in front of you, read it aloud. When you go to your appt, tell them you are really nervous. If needs be, write down why concisely/as breifly as poss in a note, pass to nurse if you feel too nervous to say it or feel overwhemed when speaking.
They will totally get it, and are better served to know how to help you in that moment.
Treat yourself after the phone call, to anything you like. praise yourself, feel releif at a task completed. I know more worry drums up after this, thinking about attending etc, but you need to do this for you. You can do it, for you.
When its done, bloody treat yourself again, cake, pizza, a drink, day out, cinema, anything. We deserve it.