Boland92 saidI talk very low and people often have trouble understanding what I'm saying. I especially feel awkward when someone leans their head in to hear me speak. I need to work on speaking up, but my self esteem is pretty low.
I presume you mean low in volume (soft) rather than low in pitch? I was shy as a kid (yes, me), and also spoke softly until I was 20. Two things changed that, to where today I have a very strong voice, and my problem is instead toning it down.
One was having leadership positions in the Army. My continued career depended on me becoming forceful and losing my shyness, so I just made it a conscious goal and priority.
But it was also easy in a way, because most of the other guys around me had loud, aggressive voices, so it was somewhat like what they say about learning a foreign language. The best way is not from a book or in a classroom, but living in a place where everyone else speaks that language. Well, these guys spoke loud Army, and I learned from them.
Second, I took a college degree in Speech, which included classes in public and dramatic speaking. At the same time I secured a position as a radio DJ, and did other voice work.
Now you probably don't wanna join the military just to improve your voice, or devote an entire college degree to it. But you could take some elective courses. And you might try hanging out with those friends who have the personalities and voices you would like to have, too, guys with whom you feel comfortable, and try to study and emulate them, just like I did in the Army. Although these might be considered bandaids, I believe that such a backdoor approach can also serve to increase your confidence, which, as you sense, is likely at the root of your problem.