The missing pieces of English pronunciation lesson

I’ve been thinking about my English pronunciation issue since I’ve moved to Bay Area (from Thailand) in 2012. I can’t seem to speak English properly.

There were many moments that made me feel bad about myself.

I can’t pronounce the word guitar. I don’t get it, and people don’t understand me when I pronounce this word. I once said guitar to a lady at the post office. She didn’t get it. I pronounced to her like several times in different stress patterns. She was very patient… but she still didn’t get. Eventually, I spelled it out for her. She got it and pronounced guitar back to me. I felt like that was exactly how I pronounced it.