Hi there Viper and Welcome to Healingwell!
If I understand this right, although you have 2 daughters only 1 lives with you and your g/f and she is a baby? I cannot honestly sit here and preach about intimacy problems and children since I have never had a g/f nor have I any children. However, I understand from others that being intimate with your partner with a new baby in the house is often difficult since your thoughts are often pre-occupied with the baby and there is the question of Post-Natal Depression. But if I understand the situation correct, your partner is not genetically linked to the baby however in moral terms, it is you and your g/f's baby. I know this sounds daft but I know it happens SO MANY times... is there a jealousy issue? Your g/f had you all to yourself and now she has to share your time with a baby. For some people, this can be a big adjustment... especially realising that the love you have for a child is different to the love you have for a partner.
Is she on any meds or does she see a councillor? It might be worth her having someone completely independent to talk to so that any issues such as her past abuse can be ironed out before the problems escalade. Catch problems while they are mole-hills... mountains are far harder to move.
All the best