I can sort of understand this because I find it very hard to tell T I don't want to do or talk about something, usually because I fear hurting her feelings.

Maybe you could say 'Just don't think mindfulness is for me at this time'.

I agree with the people who said it's worth thinking about why you don't want to do it. Mindfulness has been shown to help a great deal in many cases and even if you don't like the concept per sa some elements of it may prove helpful in your life.