I was like this and at my worst point was three stone overweight. I've been trying for the past two years to lose it and the most I lost was a stone before putting it all back on.
I started again in February of this year and have managed to lose almost two stone. I am determined to keep it off this time and also to try and lose that final stone.
Honestly the only thing that worked for me is sheer willpower. If you want to do it, you need to force yourself and not give in to the cravings.
The first couple of months are the worst but after that, I found that new eating habits have bedded in and I'm just not interested in gorging myself on junk food.
I had a 'fuck it' mentality for a long time and now I realise that I cannot do that anymore, for the sake of my health (this has been a big motivating factor).
I also do a hardcore gym class three times a week which spurs me on as I don't want to put myself through all this physical pain, only to ruin it by eating crap.
There is really no easy way around this, you've just got to dig really deep, accept that the first couple of months are going to be shit and push on through.