I'm sorry to hear about Kirby!

I understand how hard it is to put down a pet--it's not easy, but you will get through it. I promise!
I think you did the right thing by contacting your doctor regarding your prescriptions. I don't see that as sliding back, but recognizing that you need a little extra help and support during this stressful and sad time. I'd give it a couple of days once you are back on your old dosage. You'll probably start feeling better, and wanting to continue on your plan.
Start out slow--going for a walk to give you some time to reflect, or whatever you do to bring yourself back into focus. Start tracking your food again, to see where you can refocus your intake, and don't beat up on yourself!
Remember, it's a choice to slide back into old habits, so choose carefully!

Good luck!