It would be “I used to be fixated on...”, not “I’m used to be fixated on...” It’s perfectly natural to use in conversation. Note that “fixate” usually has a negative connotation, unlike “focus,” which is neutral.

The meaning is ambiguous. I’m used to fixating on losing weight. (Better than “I’m used to being fixated...” That’s something that I’ve done habitually.) I used to be fixated on losing weight. (I was, but now I’m not)

You can say, "I used to be fixated on losing weight" and, "I used to be fixated in making money". Both sentences I'm change to I. On change to in

Hello Emma! You can say: "I used to be" fixated on losing weight. and "I used to be" fixated on making money. "I'm" is the contraction of "I am" and this is in the present tense. "used to be" is in the past tense so it is necessary to change "I'm used to be" to "I used to be" because in this situation you cannot have two different tenses in the same sentence. You could also say: I used to be "obsessed with" losing weight. and I used to be "obsessed with" making money. All these sentences would sound natural in casual conversation. I hope that helps!