So I've seen somewhat similar threads out there about this topic but they didn't entirely conform to my particular situation, so I apologize if this is has been done and redone before but I'd love some advice.

I'm afraid Layla (now 13 weeks old) may never be confident enough to go for walks or a jog. I've tried for several weeks now to get her used to the traffic on our street. We've gotten most of the way down the driveway on leash and using high-value treats but she will only come to me or follow me when there are no cars going by on the street. If a car comes by while she is coming towards me she will sit or, worse, lay down and not budge or turn around and run toward the house and we have to start all over again. I feel like we've made no progress as this has been happening for several weeks now.

For a couple weeks now I have tried driving her to a small park down the road from our house that is on a much quieter street. She runs around the park just fine but when I take her across the street onto the sidewalk and walk just a little ways down the street (to get her used to it and build her confidence, again, using treats and praise) she wants nothing but to turn around and run back to the car as if it's home. I have to drag her to go anywhere, which I would rather not do because that would teach her that walks are scary and bad. And if a car comes by, forget it. She lays down and won't budge just like at home in the driveway.

I wonder how much of this could be due to the fact that puppies have that den mentality that they won't venture too far away from home until a certain age. Another part of it I know is that she is scared of cars on the road, which is understandable, but she will need to get used to this as we live on a relatively busy road and to get to the sidewalk we have to cross the street. I can't be driving her places to go for a walk for the rest of her life.

I feel like I've done everything right here. I don't coddle her or praise her if she looks scared. I only praise and treat when she is showing confidence. I take it slow and let her go at her own pace but I don't feel like we're making any progress; in fact, sometimes I feel like we're going backward. I've been at this now for a month and change and am becoming incredibly frustrated, no matter how much I try to make it a positive experience for her. What am I doing wrong, or is there some kind of explanation for this behavior that she will outgrow on her own? I don't want a fearful dog who will not confidently walk/jog with me and my husband!

I should add that INSIDE our own cars she is amazing. She climbs right in and settles down for the entire ride. She doesn't show much interest in looking out the window or anything but isn't antsy or fearful. So it is definitely the fast movement and sound of the cars and trucks on our street that is scary to her, not being in a car.

My last dog (a goldie) had this issue and it does get better! The breeder we got Indy from was situated next to an orchard so from the time she was born she was subjected to the noise of the bird scare in the orchard (Massive bangs that go off regularly to scare the birds away from the crops) so as a consequence she was terrified of loud noises for her entire life and this included cars driving past.
The main thing is to take it at her pace, a few weeks isn't a lot of time to get over something she is scared of (even though to you it probably feels like an age!) be prepared to be working on it for a long time to come. Indy was about a year old before walking was fun not stressful and all through her life even though she became fine with cars, if a big sheep truck or tanker went past us walking she would still press into my side until it was past.

As a pup I would walk down the road with her until a car went past - she would then stop dead and cower which I would ignore then as soon as the car was past I would pop her on her feet, take a few steps in the direction we were heading, treat and praise like crazy then turn right round and head home. As she got older we gradually went further and further after cars before turning around and heading home. She slowly acclimatized to the fact that scary cars don't stop us and she realized that they aren't quite so bad.
I also used to just sit on the footpath with her and let cars go past, treating before she started to tense so she was distracted and couldn't fixate so quickly.

It might help to start teaching her to make eye contact so she has you to focus on and remain calm.

It will get better just keep trying