Hi Donna,
I am also a big girl,5'8",198 lbs,down from 245 lbs.I have used a combo of strength training,lots of cycling,and am now training for my first tri.I just got my certification as a personal fitness trainer and read and study voraciously on all aspects of fitness.I applaud your perserverence,don't ever quit,it is just a matter of finding the right combination that works for you.Being a neophyte trainer,all the info I have been studying shows that when we do cardio,the fat burning and weight loss effects are immediate,what you burn is what you burn.You might consider adding strength training to your routine.When you strength train you burn calories during your workouts and after your workouts,when your body is in recovery and healing the small tears you create in your muscle tissue when you lift.You might also try zig zagging your daily caloric intake,i.e.,one higher calorie day,two lower calorie days to bust through a plateau or jump start your metabolism.Check out Tom Venuto's "Burn The Fat,Feed The Muscle" website.I purchased his e-book and that is what started this wondrous transformation for me.I hope this gives you some new ideas,and best of luck,you will find the right key to get you where you want to be in your fitness journey!!!