The I sound again has neutral, relaxed lips but this time the front of your tongue is raised a bit further - to just under the fully raised position for the long EE sound. Try moving from the high EE position to the slightly lower I position:
EE/I EE/I EE/I
Can you feel that slight but vital tongue movement? And can you hear what a difference it makes to the sound?
“I” is one of those sounds that it is better to avoid lengthening too much - otherwise “sit in the seat” could sound more like “seat in the seat”.