The term single-minded is a synonym of the following:
determined, committed, unswerving, unwavering, resolute, purposeful, devoted, dedicated, uncompromising, tireless, tenacious, persistent, indefatigable, dogged, pertinacious
Look up any of the these words in an online thesaurus and you will also find its antonym(s). For instance, the following are antonyms of determined
Antonyms for determined:
irresolute, weak-willed, pusillanimous, yielding, changeable, flexible, indefinite, soft, undecided, unfixed, hesitating, vacillating, wavering
However, in the OP's example sentence, single-minded modifies the noun fanatic, it would be highly unusual to describe oneself as a ‘wavering’ or ‘soft fanatic’. It also appears to me that the connotations for fanatic have become increasingly negative in recent years; in fact it is often used in connection with fundamentalism and acts of terrorism.
Senior cleric blasts mosque leader for praising terrorist fanatic ...
Dramatic moment robot checks the body of an ISIS fanatic for explosives after he was shot dead...
Israelis 'more prone to fanaticism and fundamentalism'
Despite the fact that fanaticism exists in world religions, there has been a strong tendency to associate Islam with fundamentalism and fanaticism.
Therefore I would employ any of the following antonyms for fanatic, which are: moderate, and conservative. If the OP wishes to emphasize the diametrical nature of the statement, I would suggest the adjective tolerant and any of its many synonyms: open-minded, forbearing, broad-minded, liberal, unprejudiced, unbiased, patient, long-suffering, understanding, forgiving, charitable, lenient, indulgent, permissive, easygoing, lax, laid-back. The majority of these would fit nicely in the blank.
You may very well think that I am a single-minded fanatic, but I am in fact just as complex and tolerant as you are.
Source: Roget's 21st Century Thesaurus, Third Edition