Part of my process is to differentiate emotional states out of the grief and the memories. Each of us is different, and we cope with loss in many ways, and if I remember correctly you were very stressed and were not prepared for this to happen although if I am not mistaken knee deep in care and details? Is that correct? Or am I mis-remembering? My Dad went over 10 years there was lots of stress and my schedule got almost knocked out of whack entirely. But because I had experience with other relatives and Alzheimer's I knew there would be slow robbery of life, and all the dear bits, and if I did not let go of the parts taken yearly, I would not grieve them and would be unprepared. So I set about letting go of the things he couldn't do or think quickly and as each shared activity came to an end, I had a new one lined up to focus on. The loss and grief were gradual, and measured.And when he left us,there was a wisp of a man who was mostly smoke. The things I loved had passed and were buried, and as many of them as I could marked with a last time. I told my sister to do the same but she didn't want to give any of it up and so, is still not right because she wants closure on the past she did not experience and that is regret and grief. I feel awful for her.
I think you were so burdened with care and responsibility you didn't get time to enjoy the parts of her that were still enjoyable or not as much as you might have, and the work was mentally and physically exhausting and instead of a reward at the end of it, like with most projects, there is absence and sadness (your sister as I recall you were concerned about keeping away from the worst of it out of care and concern). This leaves you with what appears to be no clear path forwards and the person who was always there to make that path for you is gone. But she also isn't. Your mother as she was dying is not the same woman who raised you. The woman who raised you is in you, and you demonstrated that by taking care of her affairs and your sister. You have been focused on her, and not yourself, in the selfless way we do for those we love, but she is in you and in your sister and forever and always. You can still talk to her. You can still remember fondly the things that are fond. They never fade. But when each memory is grief laden, there is a depressive element taking hold.
You say she took joy in candy giving. Yet it brings you grief. I am not wanting to invalidate the response, it is perfectly normal in exhaustion to be depressive and grief stricken. I would say to replay Halloween when you can if you can in your mind, focused on her joy. Hear her laugh, see her smile, let those feeling come over you. You may not be ready to yet, but what the grief councilors are saying is that you are repressing the trauma and it is coming out here and there. They worry its harmful for you, and it is. But it is your choice to deal with it in your way. Life is busy, there are reasons you give that are valid why now is not a good time, but when is a good time? I am going to suggest that you might consider the following (or reject it) that in some sense, the grief is standing in for your mom, and that you are holding on to it as people do not wanting to lose her again. I would say the reason you are feeling overwhelmed is because your emotional psychological self is carrying an unhealed trauma that is breaking out of you and causing trouble. You're asking for practical tips and that is good, but there are no practical tips for suppressing grief for long periods of time. It's never the right time will become it's the right time at some point. But the reasons you give are potential problems in progressing are ones that are valid to you, but they also deflect from the issue of not dealing with your grief which is hurting you.
Letting go, especially with professional help, will not make you more upset, it will allow you to move on and let go of pain. Your therapist may not be a good fit, always advocate for better if you need it. The DEEP is where you need to go because grief repressed is not at the surface. You dislike them in this instance because they are getting at the part you are protecting. What this is telling you is that part of the way you have been doing things isn't working and it needs a fix.