Since I am not there to taste your cake, and I also don't know everything there is to know about cake chemistry, I may be wrong, but...here is a guess on why your cake may have been too coarsely textured: overbeating. A cake batter that is mixed too long and hard in the mixer comes out tougher and dryer (I think this must be because the overmixing develops the gluten, which you do NOT want to do for a cake).

If that is your problem, you can solve it by cutting back on the mixing time. Some recipes call for pre-beating sugar and butter. You can beat those two ingredients together for a long time to get a nice fluffy result and your cake will be good. But as soon as you add the flour, be careful and don't mix for too long; stop as soon as all the ingredients are well incorporated.

Substituting cake flour for regular flour can also help, as cake flour is lower in gluten. However, it is not a one-to-one substitution: it is a tablespoon or so more or less per cup. I don't remember, but if you get some cake flour, I bet the box will say how to substitute it.

I hope this helps.