What's the best treatment for my underbite? I don't want any major surgery, just perhaps filler to even out the asymmetry?

jesskate83.  I understand your concern.  I recommend conservative lip augmentation with Hyaluronic Acid,  like Juvederm Ultra plus. Another option is fat transfer which is more permanent.  It may help camouflage your upper lip region.  Seek out a board certified plastic surgeon in your area, Good Luck!

You would get a nice improvement with lip fillers. These fillers, (Volbella, Restylane, Juvederm) are good to create a full and symmetrical lip. Nova threads can be used to improve lift and shape. Implants or fat are alternatives. I recommend getting a formal evaluation with a facial sculpting expert to go over your desired results. Best, Dr. Emer

Thank you for your question jesskate.  Your perceptions are correct with regard to your lips.  I assume you have been evaluated for jaw surgery based on your statement.  Using lip filler(of any type) is a great way to camouflage the profile differences.  This doesn't address the problem directly but will camouflage it by adding balance to your facial profile.  It would be best for a board certified plastic surgeon familiar with your jaw condition to provide the service. 
Best of luck.

Thank you for your question and for sharing your photographs.  Based on your narrative and photos, you may be a good candidate for lip augmentation with a hyaluronic acid based dermal filler such as Juvederm.  Beautiful and natural results can be achieved.  Please be sure to see an expert for an in-person consultation to fully discuss your goals and expectations in order to determine the best treatment plan.  Hope this helps and best wishes! 

Hello,Fillers can do great job in improving lip asymmetry as well as adding volume.Understanding of Face Sculpting and anatomy, helps to improve your concerns.Lip fillers (Restylane, Belotero and Juvederm) all would be suitable filler for this job. They last 6-9 months. Hope the BestRay Bayati, MD Elite Medical AestheticsSacramento Area, California

Thank your for your question.  Fillers can help add fullness, volume and correct asymmetries.  You can have Restylane, or Juvederm injected into or around the lips.  Sometimes Botox above the lip can help roll the lip up as well.  Look into injectors with a good reputation in your area.  Fillers are temporary and a good non surgical option.

 Hi, I have performed many facial shaping procedures using dermal fillers, facial implants (cheek, chin), liposuction and/or facelifts for over 30 years.  From the photos, your chin appears weak and the upper lip is a bit thin compared to the lower lip. Following my beauty principles, the lips should be full with the upper lip approximately 75 % the size of the lower lip for the lips to appear balanced. The upper lip can be augmented using a dermal filler or a Permalip implant.
 Dermal fillers and lip implants offer a much more reliable volume for augmentation of the lips than permanent fillers or fat injections which are associated with increased, unwanted, side effects. 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 segments of bone).  I perform chin implant surgery in 30 minutes or less, often using a local anesthetic alone.  Dermal filler cal also be used to augment the chin although is far less cost effective, over the years, than a silastic chin implant.
Hope this helps.