In English:

Reason for Refusal:
You applied for a 6 month visit visa so your girlfriend could see if she liked being with you, and the country. The Fiancee visa is designed for girlfirend/boyfriend (hence the word fiancee), and includes 6 months so your girlfriend could see if she liked being with you, and the country.

Therefor you applied for the wrong visa, hence the high refusal rates for visitor visas for girlfirend/boyfriend.

You DO NOT have to prove she will return with a fiancee visa, that is the difference, otherwise it is very hard to prove for relationships.

In addition: You are married and applying for a visitor visa for a mistress! (That is the way it looks to an outsider)

Ways around it: Only one, marry in the Philippines.