Can mouth breathing since childhood affect facial symmetry? Can it cause long face syndrome? (Photos)

It is difficult to give you complete diagnosis based on these photos. A thorough clinical evaluation by a maxilofacial or craniofacial surgeon, orthodontist and ENT surgeon is needed to establish right diagnosis and plan of treatment. You may start with orthodontist and malocclusion? Good luck.

 Hi, I have performed many facial shaping procedures using dermal fillers, silastic facial implants (cheek, chin, lips), liposuction and/or facelifts for over 30 years.  Following my beauty principles, men look chiseled and handsome with angularity in the cheeks, chin and mandibular angles.  From the photos, your cheeks are flat anteriorly (in the front), the chin is very weak as are the mandibular angles and the nose is tilted towards one side with a wide nasal tip.  The lips are thin and malocclusion appears to be present with an average degree of facial asymmetry noted.  This combines to create an elongated oval shaped face.  In my humble opinion, these issues are "real" and I would not characterize you're seeing them as evidence of body dysmorphia.   The following procedures would create a more ruggedly handsome face.
Cheek augmentation with cheek implants to add angularity to the cheeks and mid face.  
Chin augmentation, using a chin implant, to add projection to the chin creating harmony and balance to the lower face.  I have performed many facial shaping procedures, including Chin Augmentation with dermal fillers or silastic chin implants, for over 30 years.  When the chin is weak, this creates an imbalance making the nose appear larger, the mid face top heavy, the lower face looks short, de-emphasizes the lips and allows early formation of a "double chin".  Proper placement of a silastic chin implant adds forward projection to the chin thereby creating harmony and balance to the lower face.  Using the same incision, liposuction can be performed to reduce the fat and further shape the neck.  Excess skin, from below the chin, can also be removed through the same incision.  I have found that placement of a silastic chin implant, through a small curved incision under the chin (also allows excess skin removal) to be very safe, quick, highly effective and far less invasive than a sliding genioplasty (requires extensive tissue dissection, bone cuts and placement of metal screws and plates to secure the cut bone segments).  I perform chin implant surgery in 30 minutes or less, often using a local anesthetic alone.  In my opinion, you are a good candidate for chin implant surgery.  
Augmentation of the mandibular angles using a dermal filler (jaw implants have too many unwanted side effects IMHO) to add volume, angularity and flare to the jaw line.
Hope this helps.