Hmmm...not hearing or seeing you play will hinder response, but here goes:
1. Always be sure the left-hand index finger is raised for D, and for E-flat above it as well (down again for E). How's your right hand position? I've noticed, especially when inheriting new, non-beginner students, that occasionally, a student's thumb position under the flute may cause agility issues.
A recent tweaking of an advanced senior's right-thumb angle ironed out a bumpy technical spot in her latest solo (Ibert's
Jeux Sonatine), allowing her pinky to better reach multiple keys in quick succession. A good position for the right thumb is on its side, not flat. Cup your hand, like into a "C" shape. Notice the thumb's position relative to your index finger--try to duplicate this while playing your flute. Also, be sure it's in a good position relative to the keys, someplace between the F and E keys, and not sticking out too far in the front. An alternative is to place the thumb almost behind the flute, as if pushing the flute away fom you (which doesn't really happen!).
2. Never had 'em. But my dentist would prefer I get them (I'm 35!!!!!)--have to think about that one. Some students think their braces help their tone(?!), especially after removal. They feel more space has been created between the teeth and jaw, allowing a little more resonance. As for pain, not sure how to help. Be sure not to press the flute into your lower lip--it shouldn't press, even without braces, goes along with the space thing. Maybe a little oral analgesic like Anbesol or something else usually used for babies cutting teeth might help. Just don't use so much you can't feel your lip!
3. If you've only been playing for a year and a half, your student flute, if it's in good repair, should be ok for just a bit longer. I'll second that Selmer doesn't make great flutes (excellent clarinets and saxes!), but maybe you could get it checked out by a reputable repair tech and see if it has leaks or bent keys or anything? Buying a new flute is a tedious but rewarding process that should only be undertaken when we're truly ready, and have assessed our musical goals. If you just want to play for fun in band, aren't taking lessons, and maybe won't play after high school, you still deserve a nice, responsive flute, but you sure wouldn't run out and buy the caliber of flute that a potential conservatory student would buy. Players with different goals also have different ideas about what's "inexpensive". Whereas I currently consider, say, a Muramatsu EX (about $2700) an excellent value--craftsmanship and playability to dollars ratio--most of my recreational young players and their families usually consider an Armstrong 80B (discontinued but very good and still available in places, for about $1,000) a significantly-expensive flute. I did play a lovely white-gold Brannen Cooper at a convention this spring--a bargain at just $34,000!!!!!!!!!!!!!!!!!!!!!!!!!!! I'm sure somebody out there wouldn't mind spending THAT kind of money, either. So, try some Armstrongs or a Miyazawa or two, maybe Pearl.
4. Good luck!
(No sexual undertones implied.)