Hi Jessica,
sad that you end up embarrassed (ashamed?) after interacting.
speaking louder and repeating is only likely to help if you had started out speaking very softly, inaudible or nearly. If you don't feel heard the first time then repeating the same thing is likely to repeat the same result, not feeling heard, right? why would someone hear you the third time but not the first? Are you close enough, willing to be vulnerable with, feel safe enough with any of those people to tell them how you feel and inquire about their perspective?
knowing your audience by listening and asking questions (including questions to check if others heard and/or understood what you said) can be far more important than intelligence in general, or being entertaining etc..
what sorts of people are you talking with? if you were chatting with a bunch of old school computer professionals - programmers in the 1980s for example (my bias given I worked with such types) you'ld likely be the only one talking...doesn't mean they'd be listening though..

I've heard someone talk about their family's dinner table and how everyone is struggling to talk...no one pauses for a breath for fear the pause provide an opening for someone to interrupt them.
being clear about purpose can be helpful too. if we just want to be dazzeled with data and intelligence in general we can watch jepordy or read a book. as R. Grossman writes about in his essay about being/becoming a therapist, there are subtexts in conversations. my sense is we communicate about feelings most if not all of the time. Even when we chat about facts...what i find interesting i find interesting for a personal reason - we choose among all the intelligent/interesting bizzilion of facts in the world based on our personal values, experiences and emotional pre-dispositions. sometimes listening for intent, aka listening between the lines, including
listening to our own intent can be revealing...in other words are you really saying what you believe you are? What is your subtext? What is your intent in conveying what you are conveying? Are you looking for recognition and if so to what end? Are you wanting to give and get information? Are you trying to convey something about yourself and/or the topic only? Why do yo want to be heard?
What sort of responses would you need from others in order to feel heard?Are you direct? i have a harder time listening when someone talks around a point.
Conveying too many details can be another obsticle to being heard. when i am insecure about something i may give a bunch of details hoping that i can place the listener in the situation i am conveying and get their opinion without revealing what my assessment is. if i don't say what it is i am seeking up front and they don't know what is going on they may wonder why the heck they are listening to a series of detailed events and their attention may drift.
Do these people you feel aren't hearing you look at you when you speak? Do you notice their eyes begin to wander at some point or do they just look dazed ? Are they making a loud nasal sound and listening to you with their eyes closed?
Are you bringing up heavy subjects or always just skimming the surface of things and/or doing one or the other among folks who favor the opposite?
also paying attention to what you might be communicating via body language is key too. are you making eye contact? are your arms folded across your chest? howz the ole breath?

Ah, what is that hanging from yer nose? As soon as I removed the snake tatoos from my forehead yup, you guessed it... people took me less seriously.
any of that helpful?
take care, good luck with your adventures in communication land...
acappella