Yes, flashbacks and intrusive thoughts are usually treatable, sometimes with a combination of talk therapy and medication, sometimes with therapy alone, depends on how often and how intrusive the flashbacks/thoughts are, and whether they are related to past trauma. Mental health provider could be a psychologist, counselor, or social worker - depends on their training, certifications and background. Not sure what exactly they would be called where you live.

I used to have flashbacks due to my history of childhood sexual/emotional abuse. I've been through therapy and I take medication on a daily basis. They're pretty much gone now - very infrequently they bother me in nighttime dreams. So, yes, it is treatable.