Here's a question I want you to ask yourself: What's more embarassing? Stuttering or speaking slowly? By saying to yourself ''Today I will speak slowly the whole day'' you're setting yourself an unrealistic goal. All you do is make this frusturating for yourself as you will never complete such a hard task. Set lower expectations for yourself, like speak slowly for 5 mins and then congratulate yourself. Then once again slowly increase the time as you grow more comfortable with your new speaking style. I know you want to improve as fast as you can, but life doesn't work like that. Success lies in these boring mundane small habits that people repeat every single day. Eventually those small successes will compound into a huge long-lasting result. About the embarassement issue it's seems like you have social anxiety, for that I'd reccomend CBT therapy. Look it up, you can do it by yourself. It initially teaches you how to recognize cognitive distortions and how to combat them so that you care less of what other's think of you. Just please consider doing all this, I'm telling you from my experience that this works.