Honestly, I think sleep training will make such a big difference. You don't have to do CIO as such but I remember I did a mixture of the the Baby Whisperer (pick up/put down for naps in the day) as well as gradual retreat etc for night times.
I seem to recall that once the daytime nap/s were sorted, the night time sleeping got automatically better . Perhaps because if babies don't sleep properly in the day, they are overtired and wired in the evening? Of course, in reality, with DC2 etc, the whole napping in their bedrooms gets thrown out of the window as there is pre-school pick up etc for DC1!
I'd say it would be helpful if you could have someone (perhaps a parent) around so that you can concentrate on this for the next couple of days, or perhaps coincide with the weekend and your DH can take a day or two off? Alternatively, hire a sleep training/night nanny in to help you. A friend did this and made the world of difference. A tired mum/dad is not the optimum parents, naturally, so it's worth getting into a bit of a routine with the sleeping.
Honestly, if you're consistent it may only take a couple of/few days!
I know friends who didn't sleep train and were still having kids struggling to go to sleep without their mum/dad in the room until they were dozing off until they were 15+! Unfortunately, this can have a huge impact on those kids' anxiety levels and difficulty with sleep overs and trips away with school. In fact, of the parents I know who didn't do any kind of sleep training ended up with not only the poorest sleepers long term but also the most anxious of kids.
Perhaps there is always that feeling of 'I need mum/dad around to sleep', which then translates subconsciously to 'I always need mum/dad around' i.e. a certain helplessness on the kids' part?
It's kind to give your kids the gift of falling asleep without a 'prop' (often parents become the prop).
Good luck & hang in there - you can do it! xxx