Princess you are being silly! Of course you are not a failure of a mum!
I have triplets and my older 2 DD's were only 3 and 21months when they were born, so I know exactly what you mean when you say that you feel like you're leaving them to their own devices!
Firstly - regarding the milk, have you thought about combined feeding? Half formula and half breastmilk - we had to do this as the babies weren't putting on enough weight so were put on special formula for this. Really helped though - they stayed full for ages, so maybe think about that or ask your HV?
Do your DT's have swingy chairs that they can be put in to soothe them? They were brilliant for us - we used to have 3 going at a time! Maybe you could get 1 swingy chair and a bouncy chair so that you can alternate the twins between the 2? As our triplets got older we used to alternate them between the walker, jumperoo and the DJ station just so that I could get things done knowing that my 2 toddlers couldnt accidently step on them or they couldnt escape somewhere.
When I had to feed the babies, I would find out toys for my older 2 to play with - for example their puzzles and then get them playing with them and once they are settled, feed babies. Or feeding time could be book time too - 1 DT on your left, 1 DT on your lap and DD on your right with the book - she will have to flick pages as you will have both hands full!
Maybe for your DD you could get some colouring out (pencils!), playdough, craft things and once she is happy playing - feed DT's.
Obviously I dont know the layout of your house, but we had a small table and chairs (toddler size) in the lounge so that DD's could do crafty things at that, but I could still sit and feed and watch them at the same time. Maybe you could try and feed your DT's around mealtimes too, so that you can feed them whilst DD is sitting and eating her lunch - you could leave her pudding/fruit on the side for her so that she can be a big girl and get them herself once finished lunch.
Is your DD interested in the DT's - maybe she could help feed them with you (if you're bottle feeding) or she can fetch bits for you to keep her occupied.
To be honest, stuff the cleaning etc - that can be done once all DC's are sleeping or when DH comes home.
Once DT's are fed and settled, try and concentrate on DD - do some special things that she cant do alone - special books, painting etc - then when you have to leave her to feed the DT's, you wont feel so bad.
With your DT's - do they have dummies? If not maybe its something you could try? When they are crying (but meant to be sleeping) see if you can soothe them without getting them out to cuddle them, for example just 'shh'ing and rubbing their tummies - this way you can get them both back to sleep at the same time and they will hopefully not keep waking each other as much.
Nights were the worst for us - ours always poo'd whilst being fed, so we'd feed and then nappy change (= wide awake baby) and then have to soothe back to sleep. Next baby - fed - changed - soothed. Next baby - fed - changed - soothed. Baby 1 is up again for next feed!!! It seemed impossible for a while! But dont worry - soon they will be in a brilliant little routine - we managed to get our triplets eating/sleeping and pooing in sync!
Really hope this helps Princess - I know it can be so hard at times.
Feel free to PM me any time
Kate
P.s, I second what London said - if you can, get a nanny in! I think in certain areas, you can get free volunteer help from local services for parents with multiples. For some reason it wasnt available in our area, but have heard brilliant things from others!