Pre Diabetic Diet

I've Had Hypothyroidism For Almost 2 Years Along With Being Pre-diabetic. I'm Taking Medicines For Both, Plus I'm On A Food Supplament Pill That My Doctor Prescribed After 6 MONTHS Of Me Excersising And Only Losing 5 Pounds You Could Say I Was Pretty Deppressed After That. I've Got 3 Months Till I'm 16, That's A Young Age To Have All These Things Wrong With Me! I Can't Lose Weight Easily, But I Can Gain It Like It's Nothing! I Watch What I Eat, And I've Continued To Excersise... So Far I've Lost 16 Pounds In Almost 8 Months..it Makes Me So Depressed And Upset Because I Dont Know What To Do! So I Guess My Question Is, What Is A Good Work Out And Diet Plan To Stick To In Order For Me To Lose Weight More Easily Without The Risk Of Hurting Myself Physicaly,mentaly, And Internaly?
I Know It's A Lot To Ask For, But I Reaallly Need This. I Don't Want To Be This Size (20 In Pants) For The Rest Of My Life.... My Goal Is To Be Down To A 12 In Pant Size BEFORE I'm A Senior.
Thank You All So Much!