Many years ago, I had lived together with my girlfriend for about 4 years. At that time, I didn't think about marriage but, like you, became concerned that my GF might acquire some legal rights simply by living together permanently.
Therefore, I discussed this potential issue with my lawyer. He confirmed that there is no common law marriage in Thailand and that -- legally speaking -- my GF/Partner would not be entitled to anything. However, he cautioned me of the following: If there should be a legal dispute, the Thai judge might well rule in my GF's favor (at least partially). When I questioned him about the rule of law versus the application of the law, he mentioned that he is aware of cases whereby the concept of common law marriage has been applied in Thailand.
My advice for you would be to obtain legal council to clarify your current legal position.
PS 1: The previous GF/Partner and I have been happily married and we have 2 lovely children.
PS 2: You sound like a very decent and empathic person. My best wishes and stay the way you are.
