Op don't be too hard on yourself my love. I'm sure everyone on here has, at some point, stuck some frozen junk in the oven for dinner because it's easier. We're all guilty of being too tired to cook at times.
My suggestion? Stop buying the junk. If you don't buy it, you can't eat it. I stopped buying any naughty food (crisps, sweets, chocolate, biscuits, chips, oven goodies like curly fries and onion rings, etc.) a while back and it's done me and dd the world of good. If we fancy something naughty on a Friday or Saturday night, we'll pop to the corner shop and treat ourselves to one thing. Other than that, there's fruit or ice lollies if we have a sweet craving or we can of course bake some goodies from scratch - we made banana bread the other week and had great fun getting caked in flour 
Now I don't necessarily stick to strict meal planning, but I can pretty much walk round the supermarket picking up bits on offer as I go and the weeks meals form in my head. This weeks food shop was £27.25 (receipt is tacked to my fridge!) and that included all organic fruit and veg, organic milk, lots of fresh fish, some wholemeal bread and some frozen quorn products I needed to replace.
This weeks dinners for me and dd (7):
Monday: Smoked river cobbler chowder (using sweet potato, sweetcorn and peas to bulk it out).
Tuesday: Cheesy wholemeal pasta with broccoli, carrots and peas.
Wednesday: Home made sweet potato, onion and chedder grills with green beans and carrots.
Thursday: Smoked mackerel salad with garlic bread.
Friday: Spaghetti bolognese with wholemeal spaghetti and cheese (using quorn mince and bulked out with onion, mushrooms, carrots and peppers).
Saturday: I'll have a jacket potato and dd will have pizza (it's her 'naughty' day where she gets to choose whatever she wants to eat - this week was pizza).
Sunday: Quorn sausages, roast potatoes, veggies, Yorkshire puddings and gravy.
For breakfast there's toast and fruit or porridge with currents.
For lunch there's sandwiches, omelette and toast, soup or cheese and crackers.
For snacks there are apples, bananas, carrot sticks and small low fat yoghurts.
Fizzy pop is strictly forbidden in this house (I swear the stuff gives me a touch of IBS so I don't touch it and I would never encourage dd to drink it). There are some fruit squashes but generally speaking I use Tupperware juice containers to mix up flavoured water (tap water, half a lemon, half a lime and some mint leaves - am drinking it now and it is bloody delicious!)
I was very young when I had dd and food used to really overwhelm me. Luckily my mum always cooked from scratch so I had a few basic recipes down pat but the rest I had to learn. Even now, I stick to pretty basic, easy meals that we know and love, though I ditched the oven fish and chips three times a week a wee while ago! Eating well and eating healthily is actually pretty easy once you've come to grips with it. You soon realise that giving a young child an apple will keep them quieter for longer than a chocolate bar ever would and healthier options will fill them up for longer than crappy carb loaded oven dinners.
I'll say it again - don't be too hard on yourself. There's a plethora of good advice to be found on this thread and others like it. Take it a bit at a time - small steps in the right direction 