I am struggling with Korean pronunciation and people often cannot understand me?

I struggled with this too, and a few specific sounds cause most of the trouble. Korean has tense, aspirated, and plain consonant sets that English does not distinguish, and mixing them up changes words entirely, so it is worth drilling minimal pairs for pairs like g, k, and the tense kk. Vowels are another common trap, especially the eo and o sounds, and the eu and u sounds, which English speakers tend to merge. Final consonants are released differently than in English, and there are sound-change rules where letters shift depending on their neighbors, so reading the spelling literally can mislead you. The biggest practical fix is to mimic full phrases and intonation rather than isolated letters: shadow short clips of native speakers, record yourself, and compare. Apps and YouTube channels focused on Korean pronunciation help, and Korean is fairly flat in stress, so resist adding English stress patterns. Slowing down and getting the vowels and tense consonants right does more for being understood than speaking fast.