The most important thing is to not beat yourself up for slipping from 'doing the right thing' - my mistake has always been to set impossible standards and targets, then treat myself for getting part-way to meeting them, then feel awful and set even harder targets!.
I now set my sights a bit lower - instead of shunning all alcohol I have one night a week where I can drink what I want - this helps stop myself from sneaky treats during the week. If my eating and exercising behaviour in the week warrants a pint I have it on Saturday. If my job has been utterly maddening I can have 10! I'm going to slip anyway given how much of an arse my workplace is, so why not do it without the guilt and self-loathing? It's the same with bacon... If I can have a bacon day it makes the fruit and fibre days that bit more bearable. That's just my way, I'm sure you will find your own.
Ok, it's going to take me longer to lose weight, but I'm going to find it easier if I'm not cursing myself for failing to be being an angel for every single day of the 15 weeks. It's difficult enough to lose weight, don't make it impossible
The 'team' thing is going to be a big motivator for me in keeping going, I'm sure it will be for you too, but you've already planted your marker in the ground by signing up. That was 'doing the right thing', and the next 15 weeks will have bad days, but they'll have good ones too. Every single one of your teammates will be having the same struggles, and the same slip ups - hopefully you find you have teammates who you can talk to about it, to support each other outside of match days, and to win together.
Good luck for tomorrow night, are you in Pompey League? I'm completely bricking it, but will have a big fake smile plastered over my face...