I've been using the Paul McKenna 'I Can Make You Thin' techniques and he suggests that you imagine yourself slim, and then imagine moving in your thin body and going about your everyday life in your slim body.
Now I'm the same as you OP, I've always been fat. So Paul suggests you imagine yourself just a little bit slimmer. So I focus on body parts - I imagine wearing a dress I saw today, a little red strapless dress, with a black tux jacket, heels and sparkly high heels. I imagine thinner legs, and a little waist, and a visible collarbone.
Or imagine yourself doing something you find difficult now, but which will be easier once you've lost a bit of weight. I imagine myself going hillwalking, or climbing the stairs to my flat (which is on the top floor of a high building), sprinting up and then getting to the stairs without being out of breath.
Or sometimes - when I'm really wanting to motivate myself - I imagine how great sex will be once I'm thinner and more confident.
Try taking it bit by bit, rather than imagining it all in one go.