Focus on the positives, you are healthy, you are still young and your ex sounds very reasonable. For that you are very fortunate. Have that at the forefront of your mind, not the negatives. Get a journal and write all the positives every day. It really does change your brain to be less anxious.
For the pre meeting she/he will just go through the mediation meeting and what info you need. It is a chance for you to ask questions.
Do you know house value? Is there anyone like your sister who can go through finances with you. Unfortunately you have not acted like an adult for years BUT you are and more capable than you think.
Do you know house value, his pension value, savings etc. Essentially these are added up and then split. I suspect you will have to sell the house but it depends on how big it is and what his pension is.
A solicitor will help you before actual mediation or you can go afterwards once you know the figures. Your ex may pay you a lump sum to assist with living costs until your income increase. £1000 per month living expense isn't too hard to earn, £50 per day.
Everyone learns eventually to live as an adult and deal with life and so will you. It is not as scary as you think.
It is important you don't feel bitter, your Ex seems so fair and the deserves to be able to move in with his life. You may get an enormous sense of achievement to be solo so embraced the change rather than fear it.
Have you always feared change? Keep this thread going I think it could be a great journal for you to see how much you have grown.