Ok, I may be in the minority here, but I still want to express my thoughts on this. I think what Red Panda is very true. In a way if you did really call her MOM it would mean you would subject yourself to living in a fantasy world. Wanting to call her mom expresses so many things that are unmet in your life. To be nurtured, held, loved, validated and so on. But even if the desire is overwhelming, do you think it would really help you? I mean really, really help?
I am sorry if this sounds not like what you want to hear, but I don't think it would help. If your T would agree with you calling her mom, I would even think it's a very fine boundary line. You won't be able to get yourself out of the "good" feeling of having her as a mother substitute easily. In your thoughts, you can call her whatever you want. And it may even help you to see her as your mother figure for a while. But that's it, she is a mother-figure, not your mom.
I wish I had a mom who would have loved me and held me and cared for me, but instead she hated me with all she was because she thought I was taking away her husband by seducing him into having sex with me. (At the age of 6 she told me I was a *****!) I always looked for mother-figures in my life. Or father figures, for what it's worth. I wanted to be able to call other people mom or dad. But the healthy part in me found out at some point that the reality is I never had a mom, I will never get another mom and the work I have to do is to find a substitute for the desire to find that.

One day my T said to me: "I hope this is not inappropriate, and perhaps it's out of maternal feelings for you but I would like to tell you that last session I was so proud of you!"
Wow, that hit home and I felt incredible. I knew then, that even without a mom or a dad who loved me, I can do something that other people are proud of. I felt a strong connection to my T then, but I also knew that this was all me. I did it, I made this progress, I worked hard. I am not incapable. I am worth something. It gave me unbelievable strength. I thought at the end of the session that I need more of that. But the truth was, I didn't.

As for you, I don't know what exactly it is you need, but I do know that calling your T mom would probably not be good in the long run. Let it be a fantasy for a while. Imagine what it would feel like, draw strength from the maternal input she gives you when you are with her. One day the need will fade and you will feel united with the inner child that yearns for a mom.

What goingtogetthere's therapist said is so true.. it's a process. It's not a race. You will get there. A process often means there is a lot of room for progress. And imagine how you will feel when the desire to call your T mom fades and you are free from it. It will be a breakthrough worth working towards. But take your time.. You will get there :-)

Lots of love,
Amelia