Hi, I'm not sure if I can give you any tips but I can empathise with how you are feeling. My ds was very reluctant to go to school, I still remember his school visit, he spent an hour sobbing under one of the chairs ! All through the summer holidays he was adamant that he didn't want to go, he was very nervous. I bought his school uniform and he took the M&S bag and threw it in the garden hedge (very unlike him !)
He has always been quite reserved, and emotionally quite sensitive, although not overly so. He prefers watching and assessing, rather than jumping in at the deep end. He absolutely loved his nursery, he had been there for over four years and was really comfortable. I think school for him was the big unknown, and certainly taking him out of his comfort zone.
Needless to say, he took to school like a duck to water, right from the word go. He adores school, he cries at the end of term and although he's been happy in the holidays, he's itching to go back into Year 1 in September. He's got lots of little friends and has really come on with his confidence, he was really ready for the academic side of things and his teacher was amazing. He got a lovely report and Reception has been a very positive experience.
This September, my daughter goes and she is also very unwilling (has again loved nursery). She has a very different personality, but I am much less worried.
Perhaps one tip is to stop mentioning school. I know I kept talking about it to "prepare" ds, but in fact it made him even more nervous. In the end I just waited until a few days before and then brought the subject up. His teacher did a home visit just before term started and this was really useful as he liked seeing her in his own environment. To be honest the best cure for his nervousness was just to get stuck in, and realise that school was not the big, horrible, institution his imagination had conjured up.
Good luck !