I can assure you that you aren’t overthinking this. The changes you see each time her name comes up, the softened voice softens, the distance look in his eyes, are involuntary physiological responses that suggests, he still has some emotional connection to the said person, and he might even be reminiscing his time with her.
But I have a question. Why does her name keep popping up? Is it you who always brings her up, just to see how your boyfriend feels about her, or he just randomly talk about her to you.
If it is you who always bring her up in conversation, you jsut have to stop.
If he is the one who brings her up then there is an issue.
You deserve to be with someone who is all into you, sees you as their present and future, and you shouldn’t settle until you find that person.