This may be because your native language has only one high front vowel, /i/.
The best way to work on these vowel sounds is to practice on the /ɪ/ vowel and establish a strong vowel /ɪ/, then work to make sure your /i/ vowel is distinct.
Vowel /ɪ/ is lax, which means the tongue and facial muscles are more relaxed for this vowel sound than for the tense /i/ vowel. The tongue is also slightly lower in the mouth than for the /i/ vowel, but this difference can be hard to feel.
Use the video below to work on these two vowels.
To work on these vowels, practice common words for vowel /ɪ/:
Listen to and practice contrasts - minimal pairs, and mixed phrases:
When you are ready, add repeated phrase practice for vowel /ɪ/:
is it _____ ready, time, yours, mine
with the ____ group, computer, weather
did you ____ hear, say, know, talk
Use the videos below for even more practice.