how can someone who is soft spoken defend themselves when being mistreated and bullied by others?

Judging by the way you describe these incidents--and yourself--lead me to believe you are in high school. High school is not the real world, the internet is not the real world, and the sooner you realize that the better. You don't need to say anything since you're quiet (much like myself) by nature; and that is why people will listen when you do speak. Speak eloquently, annunciate, speak using your diaphragm to make your voice louder, and always make sure you can rationalize and justify your point of view.

But in all honesty, I think the ignorance you witness at school is simply this:

You're young, you're all going through phases, and eventually you will all grow up and join society as respectable adults! So just wait it out, it's much easier.

ive been in the same position as you, I had some guys constantly mess with me for 2 years straight and that was a couple years ago, but its had a lasting effect on me and im still in a dark place

I never stood up to them, wish I had, but in the end I called the cops on them coz they robbed my bike and money and stuff

my doctor recommended I do muay thai so id have confidence and be able to stand up to people but i smoke and have no motivation so i cba, but i can imagine it helps loads. theres no better way to be prepared for bullies than working out + learning how to fight if you ask me.

Have some black cards with white lettering for print. Have sayings printed for your various needs. "Your attitude is offensive." or "You're a bully and your lack of confidence is showing." "If only you had clue." Etc.

You get offended just hand over a card with a "excuse me please." (or silence and a stern look works well) and leave.

My ears hurt lol

I earned a Black Belt. You can, too.