I have entered the information you supplied about your horse's diet into MicroSteed, KER's state-of-the-art ration evaluation software.  The report is tailored to the breed, age, weight, metabolism, and activity level of your horse.
 

Since I do not have analyses of the exact forages your mare is getting, I used average values for each type, so this is an approximation to identify some trends in the diet.  I did have all the information for both the Fibregized and the All-Phase for the ration evaluation, as Pennfield is one of KER's valued Team Members. You did not mention whether she has access to salt, but it is recommended to allow free-choice salt, whether it is white or mineralized, block or loose. 

As you suspected, Fibregized is not the right feed for your mare because the feeding rate is too high for her caloric needs, and she is currently not consuming sufficient trace minerals for optimal health. Your mare is the perfect candidate for All-Phase, as 1 pound per day (1/2 lb per feeding) is all that is needed to meet requirements. Should you choose to feed All-Phase, please remember to weigh it because the All-Phase pellets are denser than the Fibregized and may fill the scoop differently. Pennfield has some convenient All-Phase feeding cups that measure out exactly one pound; ask your feed dealer to get one for you.

If you get to the point where you think that even 1 pound per day of All-Phase is too much, KER has another option for the super easy keeper called I.R. Pellet. This is designed to be fed at 4 ounces per day alone or mixed with a handful of feed.