Rhomboids are a part of life as a blackmith [at least *this* one].
The main thin I have learned about forging is to watch what your material is doing and make corrections as soon as you notice anything is not exactly what you want. Not after one more strike, or on the next heat. Stop everything you are doing, right now, and look over the material carefully and decide what you must do to make it exactly the way you want it.
If you do anything else the problem is going to get worse, and may go beyond saving the work you have already done.
This, for me, was my main breakthrough to forging decent work. Once this becomes your main focus your work will start to improve immediately. As you gain experience you will be able to make better corrections and throw less things in the scrap pile.
I usually take a good heat, rotate my material so the 'high' edge is up, and pound it down. Rotate to the next high spot. Rinse and repeat.
The vise is also useful to correct twists, as mentioned above, while material is good and hot.
Keep swinging the hammer- it gets easier!
Dave