Hello gemma, you are really brave to join and post on here, I know it's a big step. I had bad PND with DS1 and it took me a few weeks longer than you to acknowledge it. One of the biggest reasons why women don't talk about it, or don't talk about it as early as they should is that almost everyone feels horribly guilty, and worries that people will think they don't love/didn't want their baby. All GPs/HVs/midwives understand that that's not the case and that's not how it works.
What happened to me is that I eventually went to see my GP and admitted how I was feeling. I did a questionnaire for her (which is a bit easier than answering questions in person). I'm not able to take antidepressants as I had a very bad reaction to one a long time ago, so we had to tackle it in other ways, but there are some ADs which are safe with breastfeeding. In the end I saw a psychologist every week at my local children's centre and I also saw a mother/baby therapist at the Anna Freud Centre which is near us so I was very lucky to get that for free. It took a few weeks for these referrals to come through and in that time I saw my GP every week and she helped me decide what to do while I was waiting for the referrals: she said it was really important that I got some proper sleep and that I could no longer do two nights in a row. She was very clear about that which helped me talk to my husband about it, so from that point on my husband did every other night and gave DS a bottle. DS was a bit older than your baby (about three months) but this was fine and didn't cause any problems with our breast-feeding. I also started using a creche at our local sports centre so I could have a quick swim and a bit of time to myself.
When you describe thinking about crashing your car for a moment, this is a really normal part of being depressed: they are called 'intrusive thoughts', that suddenly pop into your head. It can be very frightening, but it doesn't mean that you have any intention of harming yourself, it is just a sign that you are feeling very low and anxious. It is important to mention this to your GP/HV - if you do a questionnaire there'll probably be a question about it.
I actually did the Edinburgh post-natal depression scale myself online one night and that's what made me realise I needed to see the doctor. You might want to do it yourself and then tell your doctor/HV how you scored. Once I was seeing the psychologist, she did questionnaires like this with me every few weeks so that I could see how I was beginning to improve, which was really encouraging.
Good luck. It is a horrible experience but it is also a really common one - I have been really surprised how often it happens that if you admit to a woman a bit older/with older children that you were a "bit low" or something like that after having a baby, they will say that they were too. I think it's something that loads of people don't/can't talk about at the time but it's easier to admit to a bit later on when you've come out the other side. Which you will!