Matty said it. Do what you can, but go easy on the back. If mine's bothering me, I just do slow hypers or even better, lots of abdominal work, which works the lower back indirectly and helps to aleviate some pain from the opposing lower back muscles.
Remember that the diet at this point is the thing that's gonna give you the best results anyway, the gym is great and fun, but that hard gym work will show after the diet has had its effect.
And sure, we all f**k up sometimes. But it's a big picture thing. Not every day has to be perfect when you live this as a lifestyle. If you were cutting down for a show, and had to lose 50 lbs of offseason weight, I'd say there's a lot less room for error. But theres not a REAL time limit here. If you miss your goal by a few weeks, just diet longer. No worries.
If the diet is a problem as far as volume and guessing how many cals are in your food, here's one possible solution of many: Make a diet that covers the macros and eat the same thing, every day for a certain period, say 12 weeks or until you reach a certain weight or BF%. Yes, the food boredom sucks a bit, but you know what you're eating every day. And you can adjust up or down, depending on your goals. I do a zig-zag diet this way and it works well.
