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This question hurts me to read even. What a horrible feeling. It east to feel alone even if or when we aren't though. Depending on the person maybe just let them know you need more of them. I can relate to this feeling.
Stop doing the things you usually do for them. Eg. Don't cook for them or make them their packed lunch, go out with your friends, don't let them know where you are and when you're going to come back home. This worked for me :)