I have been using a 44 inch English girth borrowed from a friend and it fit great. Bought a new one, same size and brand, and it's near impossible to get on. The elastic has NO give whatsoever. Any tips on stretching it a bit?
Girth elastic is supposed to be stiff - an awful lot of cheap girths are far too stretchy and weak and will not only fail prematurely but allow the saddle to lift that side. They should all be triple-layered, but most are only two.
You'll probably find the girth you were using had stretched. Leather girths often do, because leather stretches, especially the thin stuff most modern girths are made from. So a new girth will often be shorter, even though both may be stamped with the same size.
Yep, it sounds like the other one had stretched out quite a bit. You can measure the old one with a tape measure from buckle to buckle to see how long it is now. If it only reaches the first or second holes of your billets then add another inch or two to allow for the stretched out elastic. Usually you want a brand new girth to reach only the first hole or two of your billets to allow for future stretch.
You could add billet extenders until the elastic relaxes a bit. We used to use girths made in England which meant high caliber. The elastic was somewhat stiff and despite a lot of use the elastic never failed.