Visa Waiver Ban
And even though you never overstayed the authorized 90-day period of entry and visitors are allowed to avail of the waiver program for multiple entries, the officer who denied you would have seen your frequent travel pattern over the past 10 years as an abuse of the program, especially with regards to your career as a pool player.
What to do next? You seem to have made a name for yourself in the world of billiards, so perhaps you should look into the P-1 visa, which can be issued to an internationally recognized athlete. An immigration attorney can help determine your eligibility for the visa.
Presumably your girlfriend is a U.S. citizen, as you also mention the K-1 fianc visa. You would obviously have to commit to marriage in order to make proper use of this visa, and that's a question that only you can answer.
The K-1 visa, when issued, allows the fianc of a U.S. citizen to travel to the U.S., and the marriage must take place within 90 days after entry. Then the foreign spouse can apply to adjust status to that of a permanent resident (green card holder).
Having a green card allows for more freedom and options than a P-1 visa, and it's valid indefinitely, as long as the holder adheres to its provisions. (P visas, with renewals, are good for up to five years.)
Be advised, though, that the K-1 visa can take several months to issue. P-1 visas can be applied for using the premium processing service available from the U.S. Citizenship and Immigration Service (USCIS), for an additional fee of $1,000. This guarantees service within 15 calendar days after the application has been received.