Why does it feel like someone stole something from me?

I know being possessive is not a good trait but my ex loved when I called her "mine". Unfortunately I even got a tattoo that reads "My (her pet name)". But after 5 years she said we are too different and she wanted to date other people. During limbo phase she informed me she had been dating and didn't see any hope for us. Why does it feel some other guy or guys stole her from me?