I don't know how to do the "conceal trigger" feature but I have to tell you that being unable to say no, for me, later led to being raped.
After that I no longer knew how to say yes, even with things that I loved.
I started just using postponements, such as "Maybe" or "Not right now" or just plain ignoring invitations to participate in anything.
But the one thing I've learned about this is, if you don't make decisions, the decisions make you. How other people react to it is their problem, so if they don't like you telling them no, and if they start berating you or trying to force you to say yes, then you need different friends.
A way for you to learn to combat your reservations on saying no, try babysitting or something, where no is required. You have to get used to saying it, sticking with it, and understanding why it's necessary.
Best of luck