What causes power loss in my 2006 Subaru Forester?

what causes lost of power when climbing hills?

You need to tell us more.
Is the transmission down shifting appropriately?
Is the CEL on?

Because you aren’t downshifting your manual transmission.

I would say OP feels car is not driving like normal. Why? Who knows

How many miles are on the odometer?
Has the vehicle been maintained at least as well as is specified in the mfr’s maintenance schedule?
For example… when were the spark plugs last replaced?

More info is needed as right now none is provided. It could be an engine performance issue or could be slippage in the drive train. First meaning a clogged converter, etc and second could be a slipping manual trans clutch or slipping clutches in an automatic transmission.

Check Engine Light on or anything? Worst case scenarios are a worn out engine or worn out automatic transmission. Or both.

It’s just trying to tell you it is old and tired, and has had enough of this hill climbing garbage.

… or it has not been adequately maintained…