Definitely got a "me too" from over here!
While I most often don't give any explanation for a "no thanks" or "yes please" (I rarely see a need to know WHY someone chooses something, so I don't figure anyone else needs to know why I make a choice), there are times when a close colleague or friend might take a refusal to join them for a meal or snack as a personal rejection. That doesn't make for a good working relationship, so I have definitely used the "it doesn't fit in to my diet" as a quickly understood excuse! I do try to always follow that up with "I'd still love to get together with you, so shall we make it after lunch?" to make sure that they understand that I'm rejecting the food, and not them.
I'm going to go out on a limb here and assume that you are a reasonably intelligent person who already knows that there will be choices available at a Subway that could fit in to whatever your dietary limitations are, that you are quite capable of determining when a restaurant trip is something that you want to include in your day, and that you refused simply because you really just didn't want to go there at this particular time and change the choices that you had already settled on for the day.
I'm smirking a bit here (self-recognition) that your automatic use of the "I'm on a diet" excuse has a bit of quite lovely irony when you are so carefully working on changing your own mindset and approach to being long-term overall lifestyle change, instead of the usual temporary quick-fix restrictive "on a diet". I know that I found it a bit dispiriting to discover just how deeply entrenched that "normal" idea of being "on a diet" is for most of us - and how easily it pops out when we don't consciously stop it! I suspect that it might make things a bit awkward for you the next time that they offer and you DO choose to accept --- but it shouldn't take more than a "I'm learning to enjoy including different healthy options when I want them" to make it easy to accept or refuse in future.
Kudos on recognizing that it is time to start changing and updating your lifestyle, and then taking action to do so! Remember that it takes time to learn what is best for you long-term, and that changing mindsets and references takes a lot of time and patience --- and keep your sense of humour about when your "old normal" thoughts come popping back out of nowhere
Keep having fun, and finding the healthier and happier "new normal" that makes you feel your best!
Sir Terry Pratchett: "Science is not about building a body of known 'facts'. It is a method for asking awkward questions and subjecting them to a reality-check, thus avoiding the human tendency to believe whatever makes us feel good."
"The Inuit Paradox" (
discovermagazine.com/2004/oct/inuit-
paradox ): "...there are no essential foods—only essential nutrients. And humans can get those nutrients from diverse and eye-opening sources. "
SW: 258 Maintain @ 147-155