Does your local authority run assertiveness training courses? Or does your employer? Sometimes the departments of extra-mural studies at unis do to.
I sympathise, I find it difficult, too. Once (on a work course) we had to suggest topics to answer 'open' questions on if pairs. I couldn't think of anything, and the course leader suggested 'why you can't say no to your mother'. I nearly ended up in tears, and she cut the session a bit short - but it was nearly lunchtime.
It makes me laugh now, but mm thought of going on an assertiveness course after mf died. She wanted to because 'it will make people do what I want'.
Wish I'd encouraged her to go, the thought of the course leader putting her straight, hopefully in no uncertain terms makes me smile - she has both npd and bpd. 
Also, there is a book called 'when I say no, I feel guilty', which you might find helpful.
Don't know that it will help, but someone once said that being assertive doesn't mean getting your own way, just being able to state what you want/need in a given situation. To me, that took some of the angst out, because it makes it a bit more neutral.
Hope you find something that helps.