How to regain the motivation to run
Why does it matter if you "get back to" what you were able to do when you were younger? Why does that interfere with your motivation?
Answer that question, and you will be able to solve your motivation issues.
As a person ages, many things slip away. If you let those variances overwhelm you, you will dwindle to the essence of a wet noodle.
If you enjoyed running once, you will enjoy it again. Set reasonable goals for pursuing your desires.
Some people would give their left arm to be able to run again. Flat feet and joint issues create a lot of pain and increased risk of damage, making running a curse. I wish I could run, but I can still walk, so that's what I do.
If you lament the abilities you had in your youth, you will live with regrets that drag you further down in so many ways it'll make your head spin. Just continue doing what you CAN do, and motivation will come naturally.