It just dawned on me as i was talking to someone in Thai, that i have been most likely using very bad Thai structure for years, when explaining that my ex was Thai.

I have always said:

เมื่อก่อนมีแฟนคนไทย

When, i should be saying:

เมื่อก่อนแฟนฉันเป็นคนไทย

right?

The old way i said it, is it just bad form, or does it have any negative connotations at all?

Is there a better way to say it?

เมื่อก่อนมีแฟนคนไทย isn't really wrong, but it would sound more natural to me to say, เมื่อก่อนเคยมีแฟนเป็นคนไทย, "I used to have a Thai boyfriend/girlfriend."

เมื่อก่อนแฟนฉันเป็นคนไทย would be "My boyfriend/girlfriend used to be Thai". (Implying, of course, that they no longer are.)

Thank you!

"เมื่อก่อนแฟนฉันเป็นคนไทย would be "My boyfriend/girlfriend used to be Thai". (Implying, of course, that they no longer are.)" OHH!!!

I agree with Peppy.

or you can say this

ฉันเคยมีแฟนเป็นคนไทยมาก่อน

This is a little more natural speach patten.

ขอบคุณค่ะ MissFarmGirl

In informal speech Thais usually cut out as many unnecessary words as possible. I'd normally just say เคยมีแฟนคนไทย