We don't say 'I used to it' because it does not have the verb 'be' It is 'I am used to it' or I'm used to it'

I used to do it = 以前做的（现在不再做了） I am used to it = 习惯了

We say "I am used to it" to mean I am accustomed to something, and it is familiar to me. The idea of "I used to do it" has the idea that formerly I engaged in some activity, but no longer.

The difference is, one is wrong, one is right. You can't say "I used to it", its incorrect.

i used to it means,, it was happening in past but not happening nowadays for example: i used to wear uniform in my school days but nowadays i am not wearing