It feels horrible, i can totally empathise. What i have found helps relieve some of the intense emotions is acceptance of where you are right now. Accepting that it's a normal and fairly common thing to experience. Try to be kind with yourself for having these needs that obviously weren't fulfilled in childhood. It also helped me to read A LOT about adult attachment, attachment theory, the therapeutic process and just talking bout it to people on here who are going thru the same thing. Also i try to "mother" myself, i try to override the negative punishing or shaming voice inside and replace it with a more gentle tone.
None of that takes away the yearning for a "good enough" mothering experience unfortunately, but it does alleviate the pressure. Your therapist's ability to cope with these transferences and strong feelings and attachment issues will be paramount to your recovery and a big part of the work will be grief work and acceptance- extremely difficult. But it is very important that you have or find a therapist who is very good at this work.