I haven't stood up for myself in the past. I don't know how to stand up for myself other than anger and acting out of control. I don't have a lot of experience using other methods.
I don't think to stand up for myself; then in hindsight I regret not standing up for myself. I regret accepting behavior from other people instead of realizing someone was doing something I would regret stopping later, including things I do that I regret not stopping.
I haven't made a conscious effort to stand up for myself "in the moment" without anger. I haven't made a conscious effort to act today to prevent regret tomorrow, because I never thought or heard of that concept before now.
I have yet to say "no thanks" to anger, to work without the net of anger for the reward of developing the skills of respecting myself in a way that respects other people. I have to come up with a non-angry solution to develop my ability to come up with a non-angry solution.
I don't regret standing up for myself or any of the ways in which I have done so. I just regret not having other means to stand up for myself or any awareness of what those might be.