I have a jug dog whose neck is bigger than her head so a collar just slips off also she gets out of harnesses?

my dog is a cross between a pug and jack russel (jug) . I got her at 5 1/2 months from a mennotie who kept the puppy in a cage in a shed with no windows or light and she had very little human contact, I rescued her that day but she is terrified of EVERYTHING , I want to be able to take her for walks but the collar is a big issue because of her pysical build. Any help that could be given to me would be very grateful. Sierra is now 2 years old and i am very desparate to get help thank you everyone.