Are you playing on xbox 360? If so, I hate to say it, but you're screwed. In xbox, if your spouse dies or you kill them, you can't get remarried. And obviously you can't get divorced. So even if you wear the amulet of Mara and walk around all day, it still won't give you the option. So unless there is some glitch or secret that I'm not aware of, you can't get married again.
I believe ps3 works the same way. Unless you're okay with not being married, you'll have to load an old save.
However, if you're on PC, then that's a whole different story, and because I don't have one, I can't help you there, sorry and good luck!