My nose seems to be caving in at the tip... can this be fixed with a filler?

Careful injection of filler can camoflouge the groove temporarily, but will not actually fix what is causing the problem, and it may not give you a look that you like any better (it will add volume to the center and can give the whole tip a rounder appearance). It appears from the photo that you have prominent lower lateral nasal cartilages, and the groove you see is the space between the two sides. This is best addressed surgically. Also, a deviated septum could be addressed during the same surgery. To determine if your septum is deviated, your surgeon just needs to take a look inside your nose. If you have difficulty breathing and have a deviated septum, then insurance will likely cover your septoplasty. Your surgeon can request a pre-authorization before surgery to make sure. Some insurance companies request a CT scan for additional documentation of a deviated septum before authorizing surgery.  Insurance will not cover cosmetic alteration of your tip. If you were to get your septum fixed at the same time as a cosmetic rhinoplasty, then your insurance would pay for the anesthesia fee, facility fee, and surgeons fee related to the time spent on the septoplasty, and you would be responsible for fees related to the time spent on the cosmetic portion of your procedure. Typically if you combine the two, your overall cost is less than if you were to do both separately. Good luck!

The area that you seem to be describing in your picture is that there is a small cleft at the tip of your nose between the cartilages that form the tip-the lower lateral cartilages.  The best correction for this would be rhinoplasty, or surgery to correct the shape of your nose.  While fillers have wonderful applications, the nose is one area where caution is advised.  There is likely a greater risk of skin injury with a filler at the tip of the nose in particular.  You also appear to have very thin skin that may be more susceptible to problems.  While often a filler may look very good for some time, it will eventually go away.  On the other hand you are an excellent candidate for rhinoplasty, and re-shaping of the tip could lead to some nice improvements.  

Regarding your sinus problems, you need a thorough physical exam to evaluate what the real problem is.  A deviated nasal septum is just one of many things that can cause 'sinus' problems.  Generally, insurance will not pay for cosmetic surgery.  On the other hand, if you need septum surgery, it is the right time to consider rhinoplasty as the septum is often used for much of the nose shaping surgery.  Find a surgeon who is experienced and really understands how to deal with theses complex issues.