Where is my End Portal?

This is an issue of multiple flukes combined into one giant mess.

The map was originally generated prior to 1.7.10, however the chunks containing the stronghold were generated after an update. Because of this, the End Portal was in a different location than what the Eyes of Ender reported.

To resolve this, I took the seed, -4365712075075430210, created a new world in Creative Mode, and navigated to the above coordinates. When I used an Eye of Ender, it proceeded into a different direction about 100 meters away. Upon digging down, sure enough there was the End Portal in my creative world in a completely isolated section of the stronghold. Writing down the coordinates, I went to them in my hardcore world, and the portal was there as well.

In short, if you create a world in a previous version of Minecraft, upgrade to a later version, then explore the chunks containing the stronghold, there's a solid chance that the End Portal is going to be in a slightly different location than what the Eyes of Ender report. You could generally find it just by navigating the stronghold, however I was not able to locate my specific End Portal because it was isolated and walled off all by itself in my specific seed.

Try using the tactic called X-Ray.
See this link for more info.

It will be ease to spot since under the portal is a 3 x 3 pool of lava which gives off light.

Just in case the video is deleted, In the video the user digs 3 blocks down, then jumps and places a wooden fences. After this, the user jumps once again, but this time he places a piece of glowstone on top of the fence. Tada, you can now see all the caves and openings around you.

It's possible that ravine generation destroyed the portal or didn't allow it to generate. Without cheating, you don't have many options other than to keep combing through it as much as you can.