First of all, congratulations! Second, I am so sorry you are feeling so low during your pregnancy. I can't empathise with antenatal depression but I do have a 2 year old, and sitting here imaging what it would be like to feel really low, and be pregnant with twins and have my daughter at my toes. Wow. This would be so tough.
I am so glad you are booked to see a counsellor in the coming days. Having a person to talk to during this time can really help. It is also good to hear that you have support at home. The hormones play a part in your experience but sometimes, it is beyond just hormones and a counsellor can help you through this. Talk therapy is a big part of healing when you have low mood. Take some time to acknowledge the strength it takes get on this journey in the first place. Well done for getting here to the forum and getting an appointment to talk with someone.
If you need some extra support in between times, I am not sure if you have heard of PANDA (Perinatal Anxiety and Depression Australia) which has a support service as well as a lot of resources. https://www.panda.org.au or 1300 726 306
There is also the Australia Multiple Birth Association https://www.amba.org.au that has support for people who are having twins which is a big adjustment.
I hope your appointment goes well on Tuesday. Please reach out here if you feel comfortable to talk and share or even just vent.
Sending your strength,