Its not going to be easy, and you are going to feel really guilty for a while. This happened to me with Minnie. She and Tiger used to go outside for a bit during the day, then one day Tiger was hit by a car and died. This caused us to make the decision to keep Minnie inside. I felt really bad because she wanted to go out so badly, but just thinking about Tiger in the last moments before she died always ensures me that we have made the right decision to keep the cats inside. Just make sure you play with them a little more and try to provide a fun atmosphere for them. They will eventually be quite content..