I can see this from both sides. I talk to my mum every day more or less, or every two days, and she calls me if I don't call her, she in turn talked to her mum every day as well. So, we have a lot of contact compared with some mums/daughters.
Equally, I don't think that offloading at someone continuously is ok, if you were crying/distressed about relationships pretty much all the time, that doesn't suggest a healthy coping mechanism and so it may have been sensible of your mum to say- I'm finding this a bit much.
My children turn to me a lot and sometimes I don't quite have enough energy and resilience to deal with it and I think it's ok to let them know that. I'm there 90% of the time, but mums do have to look after their own wellbeing and also make sure that the support they offer is actually helpful and not just perpetuating the problem.
I would definitely turn back to her now- but surely there's a middle way between sobbing every night, and having some less dramatic conversations in which you tell her how things are for you right now. If she seems resistant or not listening or overwhelmed, you have your answer, that's not something she can do. Equally, it might be she has time and capacity now.
Mums aren't just support services for other people, they are humans in their own right and can't just be expected to suck up any amount of shit we throw at them, so I don't think she was necessarily wrong all that time ago to say she was getting overwhelmed.