There are a few reasons why your Mexican rice might be coming out hard. It could be that you're not using enough water, or that your rice is too old. Another possibility is that you're not cooking the rice long enough.
If you're using the right amount of water and your rice is still coming out hard, try cooking it for a few minutes longer. If that doesn't work, try using a little less water next time.
If your rice is old, it's probably not going to come out as soft as it should. Try using a fresher batch of rice, and see if that makes a difference.