Now is Now, Then was Then
Now, that's today. Then, that was the past. One can't be in the past or now at the same time. In your memories, thoughts, perhaps, but in reality, no.
The memories you have from the past will guide you if you let them. The past will help you remember what decisions you made, some were good ones, others not so much. If you use the past to help you now, the future would look brighter.
It would be great to have the knowledge of the past to always help us with the future. We don't, so we can only do our best to make the future a better place. Let's all work together to make this world a place that we can all enjoy and be creative! Compute that!