I forgot to mention the PMI Protection Maternelle Infantile They are health visitors and a paediatrician and they are free until your DCs are 6. They will know all the other mums of young children in the area and may be able to put you in touch.
The Halte Garderie which Bonsoir is a great idea, but it there can be a waiting list. Although it is worth just asking if they have the odd day they can take your DD in, even if it for an odd session each week. I was very persistent (but polite) in calling up and asking for a place for DS.
The other thing is a centre d'acceuil, which is the closest thing to a mother & baby group. It is usually a drop in centre open perhaps a couple of mornings or afternoons a week. you can have a coffee and chat to other mums and your daughter will have other children to play with. Be warned though, they do tend to be quieter than uk playgroups, and sometimes it may only be you and your dd, plus the staff, but it is worth perservering.
Other things i did in the desperate dark days of loneliness when i first arrived was to look at the forums such as angloinfo on the family sections and pm others parents who were not too far away and ended up meeting up. we met some good friends (with slightly older children) that way.
Also don't poo poo the expat community. As a lot of brits who come to France are retired, they have a bit more time and have grown up children and they can be a sympathetic shoulder to cry on ear for a cup of tea.
The other things which used to give me a boost was a nice long chat with a friend from home. you can pour your heart out about things which you cannot discuss with your DH.
Also, in the run up to your DC2's birth, could you not ask a family member or friend with not too many commitments to come for a few days to be with you?
Sorry you are feeling so low, but I hope that you can get some help and also there is that glimmer of hope that you will be returning back to the uk, so perhaps you have turned a corner. (((hugs)))