Clothing is almost certainly part of the solution, but I'd think about more than that in terms of identifying what went wrong.

Did you feel cold from the outset? The whole time you were walking? Only after pausing for a break? Did you have other layers you could have put on? Can you identify points at which you could have corrected the situation before hypothermia set in? Had you eaten and drank enough? Did you have a hat on?

For clothing, thing in terms of your "go" layers and your "stop" layers. It's so easy to lose heat quickly when you stop moving, so make sure you have something you can quickly throw on the moment you stop - an insulated/down/synthetic jacket is great. While moving, breathibility is more important - switching from poly to wool base-layers might make some difference, but having an additional fleece layer would make a bigger one. Keeping your head, hands and feet warm will also have a disproportionate impact on your body overall. Sometimes it's not possible to stay completely dry, but it should be possible not to avoid getting soaked to the point of hypothermia, so I'd also rethink your waterproofing strategy.

The biggest thing is just not to let yourself get cold. Check in with yourself often. Carry too many safety layers until you're confident you can avoid repeating this. Make changes to what you're wearing and what you're doing early, rather than waiting til you're soaked and freezing.