struggling to lose weight

hi,

i am looking for some help. i am quite fit and have completed 3 10k runs and the london to brighton this year. i train for at least one hour 5 days a week and have 2 days of.
i eat healthily, lots of veg and salad to fill me up. fish, chicken, red meat once a week. i try not to eat to much bread and although i love potatoes,pasta and rice i try and not eat to much especially at night. i drink 2 litres of water a day and try to eat low fat probiotic yogurts or fruit when i fancy something sweet.
altho i do all of this i find it stupidly hard to loose weight. i am 5ft6 and weigh 12st and am a size 12/14
i am obviously a heavy person as the 2 times in my life i have got down to a size 10 i was still 10 1/2 st. however to get to this i had to completely cut down my calorie intake and train twice a day morning and night for an hour and a half (3hours a day,doing cardio and weights). it is impossible to maintain that. as i often felt faint and dizzy!
my mother has recently been diognosed with an underactive thryoid.
i recently went to the doctor and had my thyroid checked,cholestrol,iron and also checked for diabetes as i always feel very tired.
all came back clear and im now not sure what im doing wrong. i obviosly am doing something not right as the only thing that has ever worked has been very very bad for me.
if i do loose a a few pound over 2-3 weeks, if i treat myself one day it all goes back on. for eg i lost 4lbs over 3 weeks, so my bf treated me to a home cooked meal of steak,veg, a small jacket potato and my treat was a bit of garlic bread and a small bar of chocolate.....put back on 3.5lbs.
its so frustrating.
does anybody have any advice for me?