That's what I was told too but I used an Agent for my Wife's Ed visa and did my retirement visa myself and whilst talking to the very helpful Immigration officers they were the ones that told us we could do this and how so that's probably the best thing to do, speak to them.
My WIfe is also a Filipina and under 50, she's doing a 6 month Ed Visa first and then will 'follow' me on my retirement visa as my spouse, not sure if this option is available for a GF though?
You need 800,000 baht each in a bank account or an income of 65K baht per month and in your account for 3 months, if you are on a 'tourist visa' you can switch from that to Non O and then retirement, do it yourself, take your time and work with the immigration office who are there to help you.