Is there a nether fortress in every seed in minecraft? Because I can't find one in mine anywhere?

Minecraft worlds are randomly generated, which means you might not find some of the things you want to… There probably is one but if you haven't found one yet, either you haven't been looking very long and hard(that's what she said), or you or you don't have one.

PC worlds go on forever, so there has to be one but it may be far away. Xbox had a update to have them spawn in every world and PS3 probably came out with that feature too. Just explore or look harder have fun!

You probably have one. The nether, as far as i've explored, is about as infinite as the overworld and i usually find several over time, like with villages. You may have some issues finding one if you turned off generate structures. Check your options and see if you did that. It takes a ot of exploring in the nether to find the things, so be patient, and you will die a lot especially if you plan to travel far, so mark where your going (for backtracking) and don't take anything with you thats too important.