Maybe it is cluttering, like someone said above. It’s common with people that have adhd, and affects someone’s ability to organize their thoughts into words. Also, in presentation it’s fine, but I think you shouldn’t be too concerned about saying umm or uhhh in everyday conversation. Though not saying it does help create a pause that grabs the listener’s attention, but also where you can breath and relax. I think you might just have an issue resisting time pressure. Meaning, you feel uncomfortable and rushed when someone is waiting for your response. I think you should be more confident that your time matters, and that you deserve as long as it takes to think about what you wanna say.