Hello QueenCopper: The Skeezyks is a walker. I walk outside to get where I need to go, walk my dog, & do walking meditation in the house. I also dislike having people either approach me on the sidewalk or (even worse...) walk behind me!

When I know there is someone behind me, even a ways back, I'll typically find some reason to stop & wait for them to pass.
Whenever I'm walking outside, I always wear a hat with a brim I can pull down far enough so that it prevents eye contact with anyone coming toward me. I can see their feet. So I know they're there. (I don't want to run into them!) But we don't make eye contact. Sunglasses can also help with this.
I also am easily startled. Any loud or unexpected noise will cause me to jump. From what you wrote, it sounds like you are even more sensitive than I am. But it's similar...
I'm not sure what to suggest here. Of course you could listen to music & wear earphones as you walk. But, if you're walking with another person, that's kind-of impolite. Plus it can be dangerous under some circumstances if you can't hear what's going on around you... approaching traffic at intersections for example. To some extent, I think it's just a matter of getting out there & walking regularly. The more you do it, the more used to it you will become. You may never be cured of your "street anxiety" (I'm not...) But at least it may lessen to a degree where it is tolerable if not comfortable.
I know you wrote that you feel you can't open up, to your walking companion, about why you're paranoid. But hopefully you can, & you have, with your therapist. Perhaps you might also consider at least sharing, with your walking companion, the paranoia you are experiencing when you're out walking. You might talk with your T about this. But, it seems to me, you could share this with your walking companion without talking about what it is that caused it to begin with. Being able to express what it is you are feeling at the time, might help to mitigate the feelings.
One other thing I do, which actually relates more to the practice of walking meditation, is to focus light attention on my feet as they touch the ground & on my body as a whole... simply paying attention to what is going on in my body as I walk. I say "light" attention because this is not an intense focusing of mental energy on the physical body. It's just a mild, care-free scanning of what my body feels like as I walk.