@Gabz that sounds so familiar to me, I've bounced around before, get started, go hardcore for a week or two, then blurppp back to the old patterns and routines that got me to where I was...
About 6 months ago I started again, this time I started slow, only made tiny tiny changes here and there, and so far I've been able to myself into a much healthier position. I still have those days where I completely fall off the wagon, however, I don't punish myself for it as tomorrow is another day I can try to be better.
My 2c would be to pick a small target and then go from there. As an example, make your goal for the week to drink more water, aim for say 0.5gal water per day for the week. Drinking water is a good one because of the flow on effects, which basically act as a multiplier for other healthy activities.
Just thinking about making changes is a good start, it's better than a majority of the population. Don't let yourself, or anyone else, give you shit about it.