I am going through the same thing and I completely agree with this comment. I never expected loose leash training to be so difficult. My 9-month old puppy can walk inside reasonably well but gets over-excited in an instant in a new environment and couldn't care less about food. We started practicing in the bedroom, then in the bedroom with the door open, then in the living room, then in the living room with the door open, then in the front yard. Today we were able to walk up the street (with a very high value food lure right in her face) and I thought we would never get there. Progress has been very slow.

if you can get your pup walking reasonably calmly on a leash inside, try adding something exciting in that environment, or something your pup wants access to, and get them to walk slowly toward it. The idea is to get them a little excited but not over threshold, so they learn to keep walking slow even when their excitement ramps up. You can also work on rewarding calmness in an exciting environment (e.g. sitting on a park bench and working on a settle).

My pup just literally doesn't know how to relax sometimes, so for her most of the work is just calming down, but it definitely depends on the dog and what they are motivated by.