Hi there - you must be exhausted! I have 4 yr old twin boys and a bad back, and that can be bad enough. I really felt for you when I read your post yesterday and have been thinking about what to say to you.
Re. your back - please see the GP and ask for a physio referral. It might not come through for ages, but you may still need it when it does. If it's really bad right now then make that clear to the GP and maybe she can bump you up the list somehow. Tell her how you are the sole carer for 3 small children and need to be physically fit. Cry if you think it will help! And if s/he isn't sympathetic, see a different doctor until you find one who is.
Meanwhile, tank up on Ibuprofen - they work a treat for me, and if I'm knackered i top up with some Anadin extra or similar supermarket brand pill to give me some extra caffeine on top! If your back is really tight at night, lie on the floor with a rolled up towel lenghways under your spine for at least 10 mins. It really stretches your back out. And have a hot bath when and a big glass of wine if you fancy it when everyone else is in bed. It's so important to look after yourself when you're having a difficult time and are in pain.
With such small boys still you won't be able to stop lifting completely, but when you do please make sure you are lifting right, i.e. from your knees and not bending at the waist. Ever! And start training your boys to come to you, not the other way around. Leave plenty of time to get them ready for going out, and don't let them see your frustration when they try to play games. Just ignore them and then grab them when they come to you to check what mummy is doing. Make sure you're sitting right next to the buggy with all the necessary shoes and clothes right next to you. Bribe them with raisins or a biscuit if necessary to make them do as you say (or a smartie or whatever else will tempt them - desperate times, desperate measures!), but stick to your guns that they need to come to you.
Encourage the little ones to climb into the buggy/up the stairs/cot whenever possible so you don't have to lift them unless absolutely necessary. Aafter I slipped a disc I trained mine to climb into swings at the playground f.ex- we chose the swings that were closest to the ground and I held the swing steady while they climbed in. If the buggy is too heavy to get down the stairs with the kids in it, then take the buggy first and then return for each boy and ask him to climb in. And ask random strangers for help whenever you can to get onto public transport etc - most people don't mind at all I find, especially wen they see you have twins.
If you inisist on this every time you go out and don't give in to grumpy objections then it will work and they'll get better at it with time. Encourage them to be kind to mummy and explain that mummy's back hurts - mine took this very seriously and have asked about this ever since.
So basically - find new strategies for how to go about your daily duties, encourage them all to be independent and to help you, praise them lots when they are good (esp. your 4 yr old) and ignore them as much as possible when they are not. Enlist your older boy as your helper and make him feel really good about himslef when he does. And leave as much time as you can for getting out the door etc - even if it means getting up earlier in the morning as it will make you feel less stressed and prone to anger. And don't do any stupid angry lifts in the heat of the moment - that's what I did and that's when my back went. It's better to be late!
Is there a twinsclub/Sure Start centre where you live? These can be a godsend so I'd really advise going to give yourself a break and meet others in similar situations. And give Tamba a call too - their helpline can be very useful as it is run by parents of twins and I believe they have a special group for single parents. Hope this helps - take care.