Never taken this particular drug before, only ever taken SSRIs like Prozac. Mianserin appears to be a tetracyclic (similar to a tricyclic).
You might find this link helpful:-
www.nmhct.nhs.uk/pharmacy/depr3.htm
If you see this link, it indicates that it's best to avoid driving until you are sure how this tablet affects you re:drowsiness etc. It does seem that drowsiness is a common side effect of these tablets, so if it's unacceptable to you, I would speak to your GP about it ASAP, get his advice as to whether it's likely to get better as your body gets used to the tablets or not.
PND or no PND, having a baby is an extremely tough time for very many couples, as the mum's life changes so dramatically, on top of the general physical exhaustion of birth and looking after a newborn, so arguments are really par for the course.
In terms of continuing bfing - if you are concerned that your dd's weight gain isn't satisfactory, I would speak to the GP or HV first to get their advice before switching to formula.
PND is surprisingly common - have read estimates from between 1 in 4 and 1 in 10 mothers suffer from it - however confident other new mums you meet at M & T may seem, there's bound to be someone who has PND, and the vast majority will be struggling with the adjustment to motherhood. If you have friends/good family that are generally supportive, hopefully they will understand if you feel up to talking to them about it. Otherwise internet sites like this are good for getting support from people who have been there. Also there may be support groups locally for people with PND - ask your HV.