Cheesyt, I almost imagine your W watching her phone all day for the birthday text from you. Cristy has a good point about talking to your DB coach about it, but I think it's a pretty powerful message being sent if you don't send her that text. If she believes she has you as plan B and does not get that affirmation from you texting her, it may rattle her a bit. It also reinforces your going dark. If you do send the text, at least wait until the end of the day to make her sweat a bit though! .
I know this is a stressful decision. What would make you happy? Would texting your W make you happy? Would you only be doing it because of fear of losing her due to not texting her on her bday? Can you just send that one msg and not get into a conversation with her? Just some stuff to think about.
Don't sweat it to much though. One decision neither makes or breaks this thing. You are doing well and being strong. Keep down that path and it'll all be good!
Me39 M11 : T13 D9 BD 5/31/16 In House S until 6/21/17 Divorced 10/5/18