When I text someone it converts it to a group text and includes my old phone number

[KEVRIC94]

Hi,

When I text my fiance it sometimes turns my message into a group text and includes my old phone number in the conversation. Whoever has my old phone number has responded sometimes and I don't want him/her to keep getting texts that my fiance and I send each other.

I go to contacts and select send message to my fiance and it still sometimes includes my old number when it sends out.

I go to my current conversation thread with my fiance and text in that thread, but it sometimes puts the message in a thread that includes my fiance and my old number.

I've deleted the conversation thread including my old number but it still happens.

It's very annoying; what can I do so that my phone will stop including my old number in texts?

I have a Galaxy S6

Thanks,

-Trevor