why are my tamales still mushy?!

My husband is from Jalisco. They do add flour to their tamales, but I don't think that would have resolved your problem.

Are you making tamales with banana leaves, or corn husks?
With the corn tamales, I don't cook the masa.
With the banana leaf tamales, I pre-cook the masa. (yum, I'm getting hungry now).

The mushiness...Did the steaming water drip down into your tamales from above?
Maybe you put too much sauce in? Maybe your sauce was too diluted? Can you tell us how you made the tamales? That would be a great help.
Usually if they are overcooked, they are too dry.
How much fat did you use? The 2nd answerer is correct in that you really have to whip the lard/shortening well, till really fluffy, and I'll add the hint that after whipping the lard, you beat room temperature water/meat cooking liquid into it, a little at a time, until the lard can hold no more of it. After that, I add the moist masa to the lard mixture.

The other thing is: Sometimes tamales "seem" mushy right when you take them out of the steaming pot for the first time. As they cool a little, they firm up. I'd hate for you to dump a whole batch of tamales, just because you didn't let them cool a little.

I frankly like them a tiny bit mushy, with the feeling of some coarse masa grains in it.

Misha - or whoever is an idiot
Yes, you can over cook tamales
if your masa isn't prepared well as in whipped correctly or not enough fat they will be very mushy. I know a lot of recipes call for really long cooking times like 2-3 hours. Mine are done on a gas range in about an hour tops. I make my own masa with lard and a bit of the gravy of whatever meat i am going to fill them with.

2 hours????? Well duh your overcooking them. If they are corn husk tamales 45 min- 1 hour max.

you need to add more flour there was too much liquid which made it all smushy and gushy and bushy