Life is not asking you to finish every mile at the speed of the first one.

The opening stretch of anything is run on novelty and adrenaline, neither of which is renewable. Expecting to hold that rate over the length of the thing sets you up to read normal, sustainable effort as failure. Different phases have different appropriate speeds, and the ability to slow without judging yourself is what lets you finish at all.

Compare your current effort against the middle of the thing rather than the beginning. Ask what pace this phase actually calls for. Then give yourself permission to run it at that speed.