I don't usually stop myself from talking out and looking stupid. I'm more likely to look stupid and kick myself about it, sometimes for weeks. I also put my value in how smart I appear to people in public, but I am not ashamed to be ignorant. Not much can be done about that. I hang out with alot of people who are vastly smarter then I. So I've been humbled from this quite abit. When I was in highschool I had a superiority complex, so I would do anything to make myself appear better then those around me. I was also quite a sexist. This was my down fall. In my 2nd year of college, I had a younger female classmate repeatedly call me out on my moral integrity, motivation, and my general lack of mental progress over the years. I'm still quite good friends with her.
So you should remind your self that you were born ignorant, and that you existing as finite and in time, so you need to grow one step/moment at a time, in your wisdom and knowledge of the world. If you stammer, who cares? Fuck the people who would put you down or think lesser of you for it. I like to think of life as a giant experiment and everyone/thing around me are my test subjects. By saying things to different people and gagging their reactions you gain more knowledge of the world, and learning is worth the torment of being socially awkward, or as I like to put it "Socially Callous". I still get mad when I look like a fool in front of girls I like, but if they don't like me in all my stuttering, nervous tick infested self, even as a friend, then I don't think they are worth the effort for forming a relationship. If strangers think you to be odd then who knows, maybe one day they will come to see the brilliance that is INTP, and if not they will loss the privilege of being blessed by your insight on what ever it is that you know.
On not knowing what to say. You should start writing out what it is that you are trying to find out, or even making close friends that you don't mind looking foolish in front of and talk the issue out. I find INFPs to be good at this role. Use that Ti of your's to help you articulate what it is that you are really trying to ask help with. Yes you and everyone on earth has a hard time asking for help, that's pride stepping in a keeping you from the knowledge that you truly desire. So put it aside for the sake of progress, and one day you will be that wizard in his tower beaming with the arcane wisdom of the ages. That's my advice.