Hi - thank you so much for all of the replies. I've had a couple of slightly better days and not feeling quite as bleak as I did when I first posted. I'd had some time off work last week and I think I spent too much time on my own, not doing very much, and my mood totally crashed.
I hadn't even thought about perimenopause to be honest, but that's definitely something to consider. Gosh that does make me feel old though.
I'm not currently on any medication. I stopped all of that in my mid-twenties (and to be honest was better for it). I was under the care of a psychiatrist when I was diagnosed and put on medication. Then I was looked after by a CPN. I came off of the medication without medical supervision, which in hindsight was perhaps not the right way to go about it, but goodness I felt better when I stopped. I was nocturnal for a while as my sleep pattern got all out of whack with the sleeping tabs and (later) tranquilisers I was prescribed. It took quite a while to go back to "normal".
I definitely think my desire for a third child is related to a feeling of loss, my children growing up, time slipping by etc. Practically I know it wouldn't be a good idea, I'm bloody knackered, money would be so much tighter, we'd need a new car, years more of childcare fees, etc. My rational brain knows it isn't sensible, but by my emotional one is mourning the passing of the baby years and I suppose my reproductive years. It's silly and I wish I knew how to put those feelings to bed.
On the surface I had a good childhood, but there were difficulties. I was bullied throughout my time at school, from primary right through to secondary - I don't know what made me so unlikeable to other children, but it was really hard. I didn't have many friends until I was in my late teens. I think this played a part in leading to me self-harming, which became a pretty serious problem in my late teens and early twenties. My parent's marriage was difficult, lots and lots of very awful fights. They still fight now, but I'm not stuck in the middle anymore. My parents, my dad in particular, had an awful temper and also very high expectations of me. I never really felt like I lived up to them. But compared to what some children have to endure, it was nothing really. I have a pretty good relationship with my parents now and I know they love my, despite their faults.
I do think I need some kind of therapy, I've been putting it off for years because the idea scares me, but I feel like things are coming to a head now. I have thought about trying online therapy, as that seems a little less daunting, but I've also had people tell me it's perhaps not the best way of doing things.
Thank you to the poster who said: "You can't change the past, but you can influence your relationship with your children and those you love today. Don't let your regrets over the past taint your future." - I've been thinking about this a lot, trying to focus on living in the moment and enjoying them, trying to be the best mother I can be (which is far from perfect obviously but still significantly better than when I'm at my worst) and trying not to worry about the future/them growing up. I don't want to waste the remaining time with them being small - they're 6 and 4 and to be honest they're just getting really interesting!
I still feel lost, but I have a bit more faith in this moment that I can find my way. I figure the best way to stop being afraid of dying is to try and make the most of the time left to me. I think part of my fear stems from how much I love my family, especially my children, I can't bear the thought of not being with them (have made myself cry again now thinking about it, so will sign off).
Again, thank you for the replies!