I press on and suck in my stomach a lot. like i mean every minute or whatever. it makes me feel thinner, as i struggle with body image even though im apparently underweight. i was just wondering if this is bad or not?

Sucking in is not harmful but being underweight may be, depending on how underweight you are. Try to keep your BMI between 18.5 and 25. For healthy diet: http://www.choosemyplate.gov/., exercise 30 minutes/day, drink enough water daily so your urine is mostly colorless, have safe sex, no tobacco alcohol weed or street drugs. Wish you good health!