purpley what others have said about co-sleeping above, it might be worth a try.
I'll describe what happened with mine, though every baby/toddler is different. My DD is 2 years + 1 month now. When she was first born we were in a noisy hospital with a rocking crib for 10 days. Then suddenly she got taken home to a silent flat with a still moses basket and just me there and just wouldn't settle at all, I think she was really scared. So I started letting her sleep on my chest as it was the only way I could get any rest. And that's the only way she would settle for months. Months and months. DD still woke me, I'd say she woke me about once an hour for the first 19 months. I was exhausted, but at least I didn't have to keep getting up, putting her down - with the co-sleeping, we would naturally just re-position ourselves in the bed and sleep again. I just had to make sure she couldn't fall off (sleep on the outside, bed against the wall and cot mattress on the floor on my side just ot be sure). I didn't understand why everyone else's babies went into cots just fine and began to sleep longer and mine didn't.
Then, at 19 months I had had enough, put her in the cot, in another room, closed the door, went downstairs and (this was once of the hardest things I've ever done) didn't go back in while she was crying. DD cried for about half an hour. Since then she's gone into the cot (now a cot bed) ever night, the door is closed, and she sleeps through until morning. Something just clicked. Even the transition to the cot bed at 22 months didn't change anything, she just kind of shrugged and went to sleep in it. I never open the door between bed time and morning, she never tries to come out. These days, everyone else's toddlers are refusing to go to bed, and getting up during the night - mine doesn't do this (yet.. touch wood..), ever.
All this is just to say a couple of things.. that co-sleeping might work and it's worth a try.. that it's all so hard when you are going through and it and are so sleep deprived but you will turn a corner at some point, things will change.
I would also repeat what the others have said about getting some help - if the HV hasn't helped (and they should help, and if they don't I'd say keep calling, pester them until they do).. try your GP and insist you need help right now.. and if you do feel you really can't cope today or any day, usually you can walk into your local medical centre / A&E nearby and they told me they have a place where people can just sit and not be by themselves, if they feel like they are going to hurt themselves or their children. That's obviously a last resort but just so that you know it's there. The phonelines (Gingerbread etc) I didn't try but yes please do call them and find out. The free nursery hours you can't apply for until your child's 2nd birthday and they begin the term after usually but I have those now and it's so amazing finally having a few hours a week to myself!!! - so please do apply for them and good luck x