I've never heard that or anything close to it. The only thing I'd ever heard about the co-Redemptrix thing is from flaming liberal Catholics who are CINOs anyway and fundamentalists while reciting their misinformation.
"I have been told by a number of Catholics that I am going to hell because I don't pray to Mary, and that Jesus won't save those who haven't come through Mary to Him."
Either you misunderstood or they don't know what they're talking about. No one but God knows the state of anyone's soul, so we are not in a position to judge where someone else is going, let alone ourselves...
Regards
As a catholic, I think you are full of it, and any priest who say such a thing is full of it too.
"I have been told by a number of Catholics that I am going to hell because I don't pray to Mary, and that Jesus won't save those who haven't come through Mary to Him."
In all my years as a Catholic, I never heard such nonsense from any priest or laity (and yes, I agree, it is nonsense). Such statements do not conform to true church teaching.