This happens to so many of us! You are definately not alone
Sometimes our mind takes a bit longer to catch up to us, try looking at yourself in a mirror next to this person (or sneakily catch your guys reflection in a window or something) to show yourself the comparison that others are seeing.
Try measurements, it's hard to deny the number you see.
For me, I just have this little moments of 'aha' once in awhile where I realize I AM smaller. Like when I thought I couldn't squeeze through the car door (car next to us was parked too close) and I slid right in without touching anything.
That's the best I have right now, it's a ongoing thing for me so I don't have a ton of experience, but hopefully others can provide better input.
This has been talked about a ton of times too, look through the past posts in this forum and you might see advice some have given others as well.