There are a few different factors that could be causing your cakes to turn out hard. One reason is that your oven may be too hot. Individual ovens (especially older models) can vary, so try baking at a slightly lower temperature next time.
You can also try checking your cake with a toothpick inserted in the middle a few minutes before the recipe says it should be done. If it comes out clean, you are good to go! Overbeating can also cause your baked goods to come out heavy or dense.
You will also want to check the expiration dates on your baking soda and baking powder to make sure they are fresh.
Finally, remember to always bake with room-temperature ingredients, especially eggs and liquids.
Hope these tips help!