I'm 19 as well and worked at a hardware store over the summer. I have the same difficulty saying certain words or numbers and actually have rounded up like you mentioned. Talking over the radio was a bit of a challenge but I would pretend like the radio had just cut out. One trick I used was that on my drive to work, which was about 25 minutes, I would sing. Not just a little bit, but for the whole drive. I would put on an album I knew fairly well and just belt out every song. By the way, I'm awful at singing. But this helped my fluency. I was much more relaxed by the time I got to work. My jaw muscles were not as tense, and I guess my vocals were just warmed up. Although this didn't solve everything, it really did help.