I'm just started to try and do this more, partly thanks to MN.
Always knew when I wasn't happy with a thing, but would be struck dumb in the moment.
What was the incident this week, if you feel able to share?
I find that online/messenger is the easiest place to enforce/reassert boundaries so far. I did tell IRL my 'ex' my opinions on something he'd done in the past, but softened it a little. Things carried on for about a week, then I decided to block him on everything He was really dodgy and had been succeeding in pushing at my boundaries sexually for months/years, but I gradually said no to the gross things because I truly couldn't bring myself to do them anymore.
He had hovered around on messenger for a long time until a vulnerable moment, or I would never have got involved with him. So then I decided that anyone who gave me the creeps would be gone from my FB, and I deleted a few. I met a new friend and added him on FB, and he started being 'flirty' on there. I told him to stop as it made me feel uncomfortable, but eventually decided I had to follow my new rule of blocking. It was a bit more difficult as I'll probably see him as part of a group IRL, but I just said 'I block men who make me feel uncomfortable, I have to because of my bipolar, but I look forward to seeing him at the group in future.
True MN-ers would say that I don't have to give an extra justification to them (bipolar) that they make me feel uncomfortable is enough, so next time I won't add that. It also means that men know I am vulnerable, if I tell them.
So, yes, its easier to do online, even to respond to IRL events there, though of course it's not ideal.
Another thing is to cultivate a sense of 'annoyance', anger. Mumsnet has helped me with this. This helps you respondin the moment. It helped me with a taxi driver who was being sleazy. I told him to stop as it made me feel uncomfortable.
You could also have an attitude that you're looking forward to an opportunity to be assertive- itching to practice the new MN ethos etc.
If you have this issue in relationships too, I recommend the Freedom Programme. Everyone there has been lovely. xxx