Do you do any clinch work at your school? A good thai "plumb" or under hooks work well to nullify a person with longer reach. Being a taller person for my weight, (I fought 170lbs, while being 6'1") I know trying to get my long arms into a locked up clinch is almost impossible.
Getting inside on a taller opponent is usually only accomplished when:
1) your opponent isn't moving at all AND you can move before they can react
2) Your opponent is moving into you and you use that forward movement to slam into him.
3) you can use terrain to limit movement and pin your opponent (IE a wall, car etc)
If your opponent is doing #1, then you shouldn't have any problem at all, #3 is almost as easy, especially if you practice with a padded wall. Let's focus on #2. Your opponents have to extend to strike, work on your parry's and redirections, Wait for an opportunity where your opponent is throwing a strike at you, use your redirection and come inside with their strike. A good example is off of a right cross -
1) opp throws right cross
2) you parry with your left hand knocking punch to your right as you lower level and come forward
3) drive your right shoulder into opponents armpit
4) Keep your head up and put your ear on his shoulder to stop elbows and headlock/chokehold attemps
5) wrap your arms around midesction, keepign opponents side against your belly button
from there you can work knees into thighs, groin and midsection, tailbone, spine, kidneys etc, as well as being in a good position for a lifting throw that would leave you in good control position to finish on the ground.
You can do similar things off of kicks.
Another option for getting close is a flurry of strikes that you can follow in behind.
My recommendation would be to look around for a school where you can actively practice techniques that you train (as RES said above), I woudl suggest a more comabt oriented school, such as a Muay Thai gym, a MMA school, or a hardcore traditional MA school, a hint on the last one is to look for a school with a good portion of police or military people in it.
hope this helps