I’m 18, I still suck my thumb and I want straight teeth. I’m not sure if braces will help? (photos)

Typically long term thumb sucking is related to airway and/or anxiety issues.  I suggest addressing the open bite with an Oral Myofunctional Therapist (OMT) which may let the teeth self correct.  Braces alone will most certainly fail and relappse.  Take care of decay issues first so they are not getting worse and work on prevention.
A DNA Appliance and epigenetic orthopedics/ orthodontics may be able to grow a bigger airway allowing everything to fall into place. It will also produce a full wide healthy looking smile.   The tongue is probably a bigger issue that the thumb from the appearance.
The upper lip can do amazing job correcting the open bite if the tongue is out of the way.   You need to work on thumb habit but the oral myofunctional therapist can help.If an orthodontist does not recommend OMT you probably have the wrong orthodontist!
Ira L Shapira DDS, D,ABDSM, D,AAPM, FICCMO
Past Chair, Alliance of TMD Organizations
Diplomat,  Academy of Integrative Pain Management
Diplomate, American Board of Dental Sleep Medicine
Regent & Fellow, International College of CranioMandibular Orthopedics
Board Eligible, American Academy of CranioFacial Pain
Dental Section Editor, Sleep & Health JournalMember Cranio Editorial Board, Journal of Craniomandibular and Sleep Practice
Member, American Equilibration Society
Member, Academy of Applied Myofunctional SciencesMember, Academy of Cosmetic DentistryLife Member, American Dental Association

Thanks for posting the pictures. Braces will help create a more harmonious smile. The question is are you ready to stop sucking your thumb? If not, moving teeth orthodontically won't accomplish any real benefit.