Ok, it's time to play the "I'm a personal trainer" card again.

Typically, the reason someone would start going to the gym and then give up in the following weeks is because in their heart they want to do it, but they're just not excited or stimulated about going - which basically means you're not enjoying the exercise you're doing, therefore you need to change it.

There are so many forms of exciting exercise out there it's unreal. I spent about 18 months doing parkour three times a week, and it's sickeningly difficult, but horribly effective at burning the pounds and shed loads of fun (especially in summer). Alternatively (as already has been suggested) various martial arts will have the fat dripping off you. Personally (from experience) I can recommend good old fashion boxing and Brazilian Ju-Jitsu. Both will have you completely fucked, but again, lots and lots of fun.

Most PT's in gyms will give you a complimentary session (though if they're anything like me, they will assume the sale and try and book you in for paid sessions off the back of it, so watch out for that), but they're a good opportunity to be guided through some weights-based training that you may be unfamiliar with that you didn't know you'd enjoy.

Personally, if I was in your position (which I was once, and it was lunchtime military fitness classes that got me hooked on training again, which it what actually lead me to becoming a PT in the first place), I'd play around with various forms of physical activity to see what stimulates you, and go from there.

As for diet, that's a massive post in itself, but the advice dished out (no pun intended) here already will definitely point you in the right direction