Help with anxious dog and going outside

So my 2 year old dog's anxious behavior around going outside is becoming so confusing to me. She a nervous dog in general, mostly with loud noises and around men. When I first brought her to my apartment she was totally fine going out for walks, but about 3 days in started struggling a lot to walk out the door.

If she's not ready to go outside there's nothing I can do to make her get up. When she DOES want to go outside, more than half the time I'll open the door, she'll poke her head out, look back and forth a few times and then turn around and decide she doesn't want to go out.

OR she'll ask to go out and as soon as I get her leash she'll immediately go sit on the couch and not want to get up.

I've had her for a little under 3 months now (I am aware of the 333 rule) and I have really been doing a lot to work with her. Sitting with door open and giving treats when she relaxes. Giving treats and lots of praise when she walks out the door and rewarding her after coming back in from going potty. Everything I can think of. She doesn't always take treats outside if she's anxious and being hyper vigilant.

I've figured out she really enjoys car rides and going to the park. She's much more at ease walking around the park than around my apartment complex or dog park there. And anytime I get ready to go to the car she's out the door without any hesitation or issue. Like she's not afraid or anxious at all. Which confuses me. Why is she anxious about going out just a few feet to relieve herself but is completely confident when going to the car?

I'd love any advice on working through this because sometimes it doesn't even seem like anxiety and maybe some other behavior issue or possibly something I'm doing wrong. I'd just take her to the park every single morning except she's starting heartworm treatment in a couple weeks and I'm going to have to limit her activity and already have been a bit.