‎‘I have been losing so much time.’
‘I had lost so much time.’

What’s the difference between them?

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The difference is when it happened.

I have been losing so much time = it is happening in present overall /this is a current situation, may or may not be happening at this very moment, but it’s happening in this general time period.

I had lost so much time = in the past during a particular time period of your life.
Note: because this uses the past perfect “had lost”, there really should be a second part to the sentence that says, for example, “that [something else happened]” or, for another example, “when XYZ happened”

I have lost so much time = time has been lost (recently) but it is not going on right now.

I lost so much time = time has been lost, no indication when it happened other than that it happened in the past. It is not going on right now.