Tough Dough

In addition to what Cynthia said: When working with a really sticky dough, it helps to slightly wet your (very clean) hands. Should your hands start to get gummy or covered in dough, take the time to clean it off and then reset. You'll be amazed at how well this works.

Also, you can sometimes knead dough on a board that you've dampened, instead of flouring the board. Sprinkle a few drops of water on the surface and rub it around- you don't want it soaked, just a thin film of water to act as a barrier.

If it is dense and tough, you did knead in too much flour. It's difficult to resist sometimes when dealing with sticky doughs. Trying to add water at this point will put you in the proverbial double bind. Whether you're using a mixer or your hands, the water will make the surface of the dough unbearably sticky. Dough will spin around and around in the mixing bowl, and by the time any water actually penetrates the dough, it will be so over-kneaded that it will be even more tough. You'll have an aha moment when you'll realize that if you add some flour, the dough will grab onto the water, and you'll be right back where you started. Definitely go ahead and proof and bake it, and let it be a yardstick against which to gauge future batches. Speaking of the future, a good practice to get into, especially with soft or sticky doughs, is kneading for 5 minutes, then stopping. Cover the dough or the mixer bowl with a piece of plastic and let the dough rest for 15-20 minutes. This lets the protein strands continue to absorb water without the stress of kneading making it difficult for them to do so. Gluten will continue to develop, and when you resume kneading, you'll see that your dough behaves very differently. It should need only two or three more minutes of kneading to bring it to the finished point.

Yes, my babka dough is usually relatively sticky. Try working a little more liquid into the dough. Then leave it to rise overnight in the fridge.