Hi crazy and everyone else!
I am so excited to have found this thread!! I'll be 40 later this year when DC3 comes along. I'm now 10 weeks. DD is 13 and DS 11 years. We never felt our family complete and after years of procrastinating do we or don't we try we decided it's now or never and we would regret it if we didn't at least try. Bingo, one time, got my BFP 2 weeks later.
To my shock I have gone through some scary emotions since seeing the BFP. My first thought was "s@%t, what have we done?". I spent the first few weeks sobbing and worrying about the disruption to our 'sorted' lives. This time round I have been very unwell, terrible MS day and night, something I never had with previous pregnancies. That and exhaustion have clouded my mind I think and it's been hard to feel positive.
Had horrible time with a midwife the other day (I live in the USA). She recommened to me that because of my 'advanced maternal age' I should go for genetic counselling, the statastics on having a healthy child did not look good etc etc. She aggressively pushed on this subject despite me being there to discuss scan results from some bleeding (all is fine btw) and saying we would proceed whatever. I Ieft the doctor's feeling distraught. My DH came straight home from work after I texted him and we had a frank discussion about why we wanted to do this again. So, I feel I have more peace of mind, and with the MS fading slightly I'm going to try and start to enjoy this pregnancy (and change doctor too so i don't have to see vile midwife ever again.) 
I'm dreading telling my DD. We are very close and I worry she will be jealous. I think my DS will be excited and am sure he will be hoping for a brother.
I do worry about the school runs in the morning, and don't quite know how we will get out of the house by 7am. I think my DCs will be wonderful when LO comes along and can't wait to see them all together. I think there is the potential for my DD to be confused as the mother though! I am a little worried will I remember everything and things have changed but I also know how time flies and will try and cherish the early years with this wisdom.
Haven't told many friends yet, just 2 good friends and my mum who asked "was it planned". Told her over the phone, I think she fell off her chair she was quite shocked. Am expecting MIL to be more than shocked and quietly think we are bonkers to be starting over.
Sorry if I have waffled!! Please keep this thread going, i thought I was the only one in this situation and it made my day to see this! 