I don't know what everyone else has said as I haven't read the thread (I know its short but come dine with me is calling 
ds arrived when dd was 2.9, not sleeping through the night(odd as that started to happen when 6 months pg), actually said during labour that we didn't need two and could give this one away
.
However, he is now 4 months and I honestly can tell you I thought he was quite nice but could take him or leave him for the first couple of weeks, felt like I might lose my temper a lot until he was about 6 weeks (felt at one point like I might just shake him to make him shut up), it was HORRIBLE and makes me want to cry thinking about it.
Fast Forward to 17 weeks and I can't imagine my life without BOTH of them. He is delightful now he is no longer brand new (it is hard those first 2 months!!), I have put effort in to spend time solo with him and with her and it is working for us for now. It is a massive bonus that dd adores him beyond what is normal. perhaps she picked up on my funny feeling towards him.
It is difficult having 2, you don't know how to love them both as they are different stages, you don't know if you can love both of them, you are tired and irritated with at least one at some point during the day. I am going to shout now
IT WILL GET BETTER, I promise, and you will love them both equally (hopefully) but you know there is no shame with talking to your Dr about this (am not sure about health visitors myself), although I would be prepared to talk to my lovely midwife. However talk to a health professional as there may be something more than you are aware of at work and it is good to have a second opinion face to face rather than just some mn opinions 
It will get better, it will I promise, and you will wonder how you could possibly have felt this way. However you aren't alone with thinking the way you do. thinking of you and hope it all works out!