Rollers and momentum

[Grog]

I have this problem with loosing my momentum rather early and then wind up spinning/grinding up the hill at 6mph - though perhaps that isn't so bad.

Indeed it doesn't seem bad to me.

Any tips on how I can maintain my momentum longer? I suspect that I am gearing down too soon - just trying to work this out in my head.

For what it's worth, I tend to pedal harder on the downhills if I see an uphill ahead. Once I'm on the uphill, I try to ease the pedaling and change gear just a second before I need to do it. Two seconds if I need to switch to another chainring. Practice allows me to do this pretty seamlessly now so to avoid losing momentum. On a 2-km hill however momentum becomes pretty irrelevant: it will help you for the first 300 meters and then you're on your own. A mantra will be more useful than momentum at that stage.