Oh boxoffrogs I completely understand your post, please know that you are not alone!
In my case it has been more anxiety than depression, but I get exactly what you mean when you say you don't feel normal anymore, and it's a really horrible, scary feeling.
I would definitely encourage you to speak to your midwife, I am hoping to find the courage to speak to mine at my next appointment too as I understand that it is not uncommon and there is a lot of support out there when you ask for it. Just a word of advice though, I think you need to be quite clear with them - I have tried tentatively mentioning it to both my GP and midwife already but I think in my area they are just so busy and rushing to see the next appointment that if you end up just mumbling something like I did "well I have been feeling quite anxious" they have said oh yes that's quite normal and sort of dismissed it and I've not been brave enough yet to say no, it's not normal and tell them how bad it really is.
I basically feel like my hormones are out of control and I can't let myself get excited about the pregnancy for the fear of something going wrong. This came completely out the blue and I never expected to feel this way. Like you, my pregnancy is very much wanted, in fact I want it so badly that I have worked myself into a frenzy about "making a mistake" (eating a forbidden food, touching a surface in the house that the cats have contaminated etc...), alongside the other constant worry that something will just spontaneously go wrong. I can't concentrate on anything else and I feel like I'm not myself anymore. I can't relax and have fun with friends and family or even pay full attention to a conversation, all I can think about is what "mistakes" I might have made that day, or the outcome of the next scan or test. I cry all the time and have been driving my poor DH mad with my constant need for reassurance and crazy hygiene obsession.
I just want you to know you are not alone and I'm sure in time we will both feel a million times better. I think pregnancy does some strange things to your brain and hormones have a lot to answer for, but please don't suffer alone! I too just want to feel happy and excited, but for some reason my brain can't seem to let me at the moment.
Sorry this has turned into a rambling essay - I wish you all the best for your appointment and please know we are here to chat if you are feeling down 