Maybe this isn't the right time for you to be dieting? I think you really have to be ready, and only you know if now is the right time.
If you are ready, realize that you may still have screwups here and there. But the key is to 1) keep those to an absolute minimum, and 2) get back on plan asap...no "well since I already blew it I may as well stay off plan for the rest of the weekend" kind of excuses.
I had a bad week last week due to a broken pipe in my kitchen, and major stress associated with my job and my kitchen remodel (which seems like another full time job!). But I picked myself back up, reminded myself how much I want to lose the weight, and am now back on plan and recommitted.
What also helped is for me to set a short-term goal...insetad of saying that I need to be on plan for the next eternity while I lose all this weight, I set myself a short term goal of 2 weeks (between now and a business trip) because there is NO REASON that I can't stay on plan for two weeks (and it seems WAY less than 4 months!).
If you really want it, you CAN do it!!