1 year old terrified dog help me

When I adopted this dog, they said that she was fearful. I was like no problem because I've had a fearful dog before. I did not expect her to be excessively unbelievably fearful. Took 3 days of me sitting by her for her to warm up to me. Taking her outside a lot of things scared her especially people during the day so I started taking her out at night because the amount of things that scare her was cut in half. A year later and I'm not feeling like I got anywhere. Things she experiences every single day she just can't get comfortable with. Yesterday I took her to Petco to weigh her on the scale outside of vetco one paw touched the scale and she pulled out of her collar ran towards the door and the customer leaving didn't even realize she was running up behind him. So she got outside and it took about 20 minutes to lure her back in. She's very food oriented and she likes to chase so I eventually got her to notice I had treats in my hand and chased the treats into the building. So I would like to know if she were to slip her collar again, what is like the fastest way to try to catch her? Should I even attempt to take her to dog training classes?