What can I do to improved my naturally tight, forced-looking smile so that my upper teeth show? (photos)

It might be as a result of your upper jaw not growing froward enough and your orthodontist tried to mask the issue to get a good bite. If so, you would need to reposition the upper jaw to fill up your smile.  Please see an orthognathic surgeon for a consultation. 

You basically have three options, so I'll list them in plain English.Option #1 - Jaw Surgery - This will enhance the appearance of your natural smile, but could affect the functionality of your teeth and jaw.  I recommend not going this route, unless the cosmetics are more important to you than the functionality.Option #2 - Facial Surgery - You could have your lip adjusted and lower face altered by a plastic surgeon, but I would avoid this option as well because you don't want your face to have the appearance of being cosmetically altered.Option #3 - You could repeatedly practice smiling like the first "unnatural" smile photo that you posted.  Eventually, you may smile like this by habit.  This is the option I would choose because even though it seems hard, it is a lot easier to learn, than recovering after facial or jaw surgery.  Good luck and I hope this helps. Follow me if you have more questions.Sarah Thompson, DMD

As the problem here is not with teeth, we need closeup facial pics from various angles and xrays to analyze your problem. Then an appropriate treatment plan can be proposed. However, from what ever little I see from the above pics, you need either a corrective jaw (upper)surgery or upper chelioplasty(lip surgery).