weight loss, helpful advice

my biggest Insecurity Is my weight, I'm always determined to lose weight I start off strong for a week, and suddenly go back Into the same habit as eating unhealthy, eating whenever my mind assumes I'm hungry, eating when I'm bored, and of course eating whatever I feel like. I have a memebership at the gym I will go for a week, and suddenly lose motivation to go & keep up. I want to be at my goal weight more than anything, I want to be happy with myself & most importantly I want to be able to go to prom this year without having a melt down over finding a dress. my most main focus Is my arm fat, I want thinner arms because you cannot go with finding a prom dress to some what or cover all your arms. please send helpful advice, tips & most importantly motivation. I have a very supported boyfriend who constantly reminds me I'm beautiful, & Im not fat although I have supportive words from him I just wanna be able to believe those words.