Well, heaven knows I'm not exactly a weightloss guru, but I'd suggest 2 things:
1) Think very hard about whether there's any psychological reason that you're sabotaging your diet. It might just be my recollection of Fat is A Feminist Issue talking here, but sometimes we can be our own worst enemies and it's on purpose - because there's something about losing weight that we find threatening.
Worth considering, just in case there's something going on there.
But my main thought is (2) you need to plan ahead. Think about the situations that are going to crop up where your diet can be sabotaged (somebody at work offering you their home-baked cookies and you don't want to offend them, you're ravenous and your boyfriend suggests stopping at a drive-in, you have to eat at a friends' house and you know they'll be making something that's off plan, WHATEVER) and figure out ahead of time what you can do in these situations. You need to be armed with some strategies for how can you avoid eating stuff that you don't really want to eat. Or how you can minimise the off-the-wagon-ness - because the thing is that these situations WILL crop up. They're hideously inevitable!
I was sorely tempted to eat some of the chocolate icecream cake that one of my kids (I'm a teacher) brought in for their birthday today - there was a freaking PIECE sitting RIGHT THERE for me, all yummy and chocolatey with a chocolate-coated cherry, and, God, it was tempting. And I thought about it, but then I thought: No, if I can either lose weight OR eat the cake, I'd rather lose weight. In fact I even said it out loud! And then sent the slice next door to the teacher in the other room, so it didn't sit their looking all seductive and calling to me in its sweet, creamy, siren song of chocolatey goodness. (Not saying that would work for you - but for me that gave me a little moment of clarity and it was enough to kick me back into resisting the damn icecream.)
It helps if you make sure you've eaten already before you go into situations where there's going to be food. If you've had a filling soup, or if you've got some cubes of cheese or something On-Plan in your pocket so that you can chow down on something - this helps.
God, I hope I don't sound annoying! But I so want you to succeed in this - I want us ALL to succeed in this! And I totally get the horrible temptation thing (God, tacos, just READING that made my mouth water!), especially when there's a boyfriend/spouse/kids in the picture to crank up the temptation to the max.
If I'm coming across as a pain, please disregard the above - honestly just trying to be helpful here.