Usually speed peaks around 24, concurrent with bone mass density peaking. Do what you can to increase your home density while also getting down to a race weight of 1-2 pounds less than you were at when going 1:45.
Physically you should be a couple percent weaker even at your best, but mentally at 28 you can be more disciplined than at 24.
If you are only 10 pounds or so over your previous weight when going 1:45, you should be in 1:50ish shape within 3 months of serious training.
How lean were you when going 1:45? How much alcohol were you drinking? How much were you sleeping? Were you in a new relationship, like less than 1 year into it? There are a lot of things that you can do to increase your vitality.