Some suggestions:

1. Using a finger should be the last option. There is a chance your nail or hard skin can cut the inside of her causing discomfort.

Instead, use a silicone based, SOFT toy. Ones designed for anal get gradually bigger towards the bottom allow you to "work up" until you get to somewhere around the same size as your dick.

1. Expand more on lube. Spit is a terrible alternative and should be avoided. Water based or silicone? Thick or thin? I recommend a thick, silicone based lube for beginners. Water based is easier to clean up but dries out too fast. Thin can feel better but can also cause more discomfort.

2. Lastly, and most importantly, is getting her to associate anal with pleasure. Before you even think about putting your dick in, she should have had multiple orgasms while something is inserted into or touching her asshole.

Try a week of regular fucking while a butt-plug is inserted. Eat her out while slowly moving a toy around.

One bonus tip: While it seems counter-intuitive, telling her to "push" like she is taking a shit will actually make the sphincter relax and easier to insert. If this fails, waiting motionlessly, sometimes for several minutes, will calm the area down as well.