Could mouth breathing be one of the causes of my weak chin? (Photo)

Your nose is not a bad one, to be honest and considering you are a male, but indeed it admits customization to fit your goals, essentially a subtle downsizing, dorsal lowering and tip deprojection (frontal view modifications need other pics to be assessed).The chin is massively underdeveloped making you an ideal candidate for a multidimensional augmentation genioplasty with bone grafting supply, so that your chin is drastically masculinized and made in proportion to your facial balance.It is true the vast majority of surgeons use the implants instead of the osseous surgery to adjust the chin, but this is a matter just of lack of experience and comfort ease; implants play no role when there is an autologous alternative which, actually, has plenty of superior advantages.
Being an option, chin implants have serious disadvantages, especially in the mid and long term, patients are rarely happy with them after 5-6 years, all are out sooner or later. A quick research just on this website sheds light on the common, frequent and serious complications and side effects chin implants entail, like dislocation, malpositioning, infection, over or under corrections, visibility, fake looks, nerve damage, chronic pain, poor aesthetic results and in general patients' dissatisfaction; not to forget chin implants are not versatile, they are unable to provide vertical increase, horizontal axis modifications and in most cases sufficient anterior projection; as obvious as it sounds but we have to bear in mind chin implants play absolutely no role when the chin needs reduction of one of the spatial axes; actually it is very common receiving patients who seek chin implant removal and replacement by an osseous genioplasty in the same procedure.
Fillers, another tempting resource, are a very dangerous option in chin reshaping, potentially destructive, unable to reshape and enhance respecting the basic principles of anatomical beauty, leading to mid and long term serious complications (granulomas, deformities, fistulae, etc) and the worst is they can't be removed in most cases.
Burring the chin is, strictly said, osseous chin surgery but should neither be performed, ever, since just flattens the chin by destroying the natural dome of the jaw, deletes the labiomental crease and due to the detachment of the mentalis tendon this technique commonly leads to a droopy double chin, a serious aesthetic complication.
A multidimensional sliding genioplasty allows to a versatile surgeon full customization for augmenting, reducing and leveling the chin in any of its spatial axes, leading to a myriad of combinations to suit every individual's chin anatomy and facial balance requirements; you can masculinize, feminize, round or make square, increase, decrease, supply bone graft, remove bone portions, slide fragments, etc. This absolutely impossible with other method than multidimensional sliding geniplasty, a technique providing a series of advantages among which worth mentioning:-lifetime procedure, no more revision or renewal procedures, unlike implants-more natural than implants, the procedure is totally undetectable -no visibility of implant edges during gestures, expression is even improved, nicer smile-improves the double chin effect at the sub-mental muscles (floor of mouth) hammock caused by lack of tension there; also improves the shape and tension of perioral muscles and tightens up the jowls, creating a beneficial side effect in form of an indirect facelift and neck lift, all patients see their face indirectly rejuvenated; implants do not offer this feature-no future displacements or neuritis of mental nerves-no droopy double chin (the mentalis tendon is not detached) -way lower rate of postoperative complications if compared with implants, like infections, displacements, neuritis, etc-no need for complex plates and screws or teeth braces that may need future removal; just steel wires synthesis may suffice, they stay buried into bone callus producing no disturbance
The cons for the patient are essentially the costs associated with osseous genioplasties, however it sounds cheap if you consider the lifetime benefits; obviously the main disadvantage happens on surgeon's side, since it is more technically demanding than chin implants, few surgeons are experienced or comfortable with multidimensional osseous genioplasties (though I personally perform 100% of the chin procedures by this osseous method); patients have difficulty in finding surgeons experienced and comfortable in osseous genioplasties able to freely modify the chin shape and size based on their aesthetic and gender goals.
You'll find in this website hundreds of negative experiences with chin implants in the short and mid term, do read my profile's Q&A and you'll find them too. I am doing osseous genioplasties for more than 15 years with the highest success and no one patient sought afterwards switching to chin implants, totally the opposite, I did many chin implant removals and one-staged osseous genioplasties to replace the implant and enhance the result.
See the link below to find few cases of my own practice very similar to yours which I had the opportunity to operate successfully on, sharing a lot of common features with the technical problem you have posted.
If you wish better grounded opinion well lit, focused and standard images have to be assessed: frontal, both lateral and both oblique views, also from underneath.
Feel free to request any additional information from me.

Odin123:
We now know from twin studies that environment has little to no permanent impact on anyone's anatomy.  It looks like you were born with a chin and jaw that are smaller than the anatomy of the upper two thirds of your face.  It is much more likely that having this configuration (and the related  inborn internal anatomy) causes your mouth breathing, instead of the other way around.  Your nose appears to be attractive, masculine, and proportionate with the upper two thirds of your face -- so procedures that enhance the size and masculinity of your chin and jaw line would make more sense to bring the whole face into balance.  There are now less-invasive methods to do just that, if the more invasive and expensive bony work is not of interest to you. 
David Teplica MD, MFA