How do i remain motivated ?

i have always been overweight throughout my life since my childhood days. i am now 21 years old, 5'7 and 75 kgs. i seriously want to lose at least 10 kgs and look good. i do diet and exercise. i'v been doing that for the last 4-5 yrs. but my problem is that i tend to break it easily. after like 2 wks of ex and diet, i'm again back to my normal unhealthy eating habits. whenever i'm about to start eating healthy, i'm like highly motivated. but a few weeks down the line, i don't know where my motivation fades away. Can you tell me how do i stay motivated so that i can continue my diet and exercise easily?