I had the same issue when I went self employed. Didn't realise or more the fact I didnt expect I would get as busy as I would straight off. Larger jobs than expected, full bathrooms etc and it tied me up. Had to cancel / push jobs back that i didn't want to. They were happy once given a date but like you say, when stressing about it it really does get you down. Yet having "too much work" should never be a bad thing.