Rules as written, Feeblemind gives a specific condition for reversal. Namely:
until a heal, limited wish, miracle, or wish spell is used to cancel the effect
So, technically, Feeblemind is an extremely potent spell that is near-lethal to any character that isn't pure melee (and even then it's powerful). This is the deliberate intention of designing a spell that is powerful with limited solutions. The spell description is extremely specific -- spells do not usually list these kinds of things. Of note is that it omits spells that normally counter ability score loss from the usual sources (spells such as Restoration).
Medium-Rare (50% Raw)?
The problem here, as stated, is that Break Enchantment is a later spell than Feeblemind, and we have a rules conflict. RAW, Break Enchantment can end effects that would include Feeblemind. The problem, however, is that as stated in another answer you have a "general." Break Enchantment states:
This spell frees victims from enchantments, transmutations, and curses. Break enchantment can reverse even an instantaneous effect.
Now, the reason this is considered a general rule is that it merely gives a broad list of effect types. It does not use an intensive/inclusive word such as "all" or "every" to emphasize this. This further supports the general nature of the spell.
That said, RAI one can arguably say that Feeblemind is overly powerful and should be grandfathered into the umbrella of Break Enchantment. Without direct clarification from the creators, we cannot say for certain, but there stands a decent case for this.
As a DM and player, I would say that Feeblemind cannot be broken by Break Enchantment. Here's why:
1) Feeblemind is specific. Generally speaking, when a specific condition is given in the text, that condition must be met...not some other condition.
2) Break Enchantement is a rather general spell and does not state "all" or the like, meaning there is room for exceptions in the specific.
3) "Going to the source" -- Both of these spells are direct (as in, literally word for word) ports from DnD 3.5e. Of note is that in 3.5e they both appear in the Player's Handbook. 3.5e is famous for contradictions, but the Player's Handbook is one of the few places that they kept at least mostly coherent. The wording there remains the same, and considering that Break Enchantment appears 22 pages BEFORE Feeblemind, it would seem that the wording of Feeblemind is designed to be a direct exception to Break Enchantment.
Thus, with these views in mind -- again, I will say that Feeblemind trumps Break Enchantment -- it cannot be broken.