Suggestion: Just try to sock away 1 hour a week. Select a topic that you remember being good at. Then just pick one chapter from one of the free books listed, and have at it.

The mind map in your head will start reconnecting fairly quickly I imagine.

I personally find the sheer quantity and range of these free pdfs daunting, so as a renegade physics graduate I'm focusing on Hammack's The Book of Proof this summer. As you did Maths at University, you might not need elementary stuff like that having already learned the strategies for abstract proof.