My underbite's coming back. i wear my retainer every night. why is it coming back? is it my bone structure? or could my retainer not be tight enough?

Need orthodontist: Depending upon how long you have been out of braces, you need to get back to the dentist/orthodontist whom treated you. If this is not possible, I would see an orthodontist asap. If it is truly an under bite issue, an oral surgeon may have to be consulted.

May be relapse: You don't mention whether you wear a retainer on one jaw or both, but the fact that your bite is changing means someone should be examining you in person - too many factors at play here - possibly your jaw structure and not your teeth - it is not likely due to your retainer not being tight enough, but you need to be checked before more relapse occurs.

Growth: At 17 your jaws have not finished growing. People with your type of malocclusion tend to grow longer and later than average. Retainers will hold your tooth alignment, but will not prevent jaw growth discrepancy. You need to find the best Fully qualified Orthodontic Specialist in your area to asses your growth and subsequent treatment.

See an Orthodontist: All individual situations are obviously different, and a professional opinion is advised in your situation. Do not delay.