I left Blackreach through the Great Lift at Alftand. I wanted to go get some more arrows before continuing on the Discerning the Transmundane quest. However, now when I go back to the lift from Skyrim, there’s no way for me to get back in to go back down. Do I have to retrace all my steps thru Alftand to get back to where I left off?

I have made this mistake many times. When you exit the lift you must open the gate, there should be a lever on the wall that will open it. Once done it should stay open so the lift is always accessible.
Retracing shouldn't be so bad if you have already cleared a path, unless you've done a lot if sleeping and waiting since leaving.

Unless you discovered the other Great Lifts, yes you will have to go through it again.

I can see how you made that mistake though, that lever does kinda blend in.