Wrap your gun in wax paper from the kitchen. Take about two or three wraps around the gun and grip and trim off some of the excess, especially on the barrel end. You want the end of the barrel covered by the wax paper but not a wad of the stuff there. Press your empty Shield into the holster as far as it will go and park it on the shelf for about 24 hours. When you remove the gun from the holster, hold the holster securely in one hand and move the gun quickly in and out of the holster several times, enough to kind of warm up the wax on the paper and transfer a bit of that to the inside of the holster. Remove the wax paper and test the gun for fit. It should be better. If it's still way too tight, wrap up the gun again and reinsert it in the holster and leave it again over night. You will want to start out with the gun just a bit snug in the holsters since it will likely continue to get a bit looser over time and use. Usually doing this will help gain just a bit of space inside your holster plus the wax will make the movement a bit slicker. Like I said, take it slow and easy because you don't want to over do this. Wear the gun and holster as much as you can for a few days and see what happens. If it's still really tight, give it another treatment.
The .45 Shield will fit in the same holster as the 9MM Shield, EXCEPT for the length of the barrel, which is about a quarter inch longer on the .45. If your holster is open at the bottom so the barrel can fully enter the holster, both guns should fit just fine. The Shield has a slightly longer barrel/slide and the grip area is slightly longer. Other wise the grip frame is the same size as near as I can tell.