Wife Masturbates Instead of Asking for Sex

[Burner account for privacy reasons]

My wife and I have been happily married for 15 years.  Sex is life is good, though I’ve always wished that she weren’t so reserved.  I’m the one who initiates almost all the time, the one who suggests that we try new things, etc.  She grew up in a more conservative, repressed environment, where sex was “dirty.  She often expresses frustration that she still has lingering discomfort with the whole sex thing, that she wishes she could open up more.  That being said, I’ve come to terms with it and it’s really not a problem for me.  However, I do, once in a while, try to gently push her a bit.  She says it's ok with me and I'm quite comfortable saying “this isn’t for me” if she’s not ok with something.  

A few years back we started sexting each other occasionally, sending nudes, etc.  At her request, I would send more graphic stuff (me with an erection, masturbating, stuff like that).  A few times I’ve asked her for some pictures/videos doing solo play, but she always said no, saying she doesn’t even do that off camera.  I wasn’t surprised; she doesn’t even touch herself ever while we’re having sex.  She knows it's something I would really enjoy, and feels bad that she's not able to do it for me. 

Last week I was out of town, and I texted her a nude.  Out of the blue, she starts telling me that she can't hold it in any longer, and “confesses” that she sometimes masturbates in the morning while I'm in the shower.   She says she sometimes wakes up in the morning with what she describes as a  “physical feeling” (not horniness per se) and takes care of it as quickly as possible.  (The sound of the shower turning off is apparently the warning sign that she needs to finish quickly).  I was rather stunned.  Not that I cared that she was masturbating, there's nothing wrong with that, but after texting back and forth, she sort of admitted that she does it simply because she doesn't feel comfortable asking me for sex in the morning (we do have morning sex sometimes, but I'm always the one who initiates).

So she is simultaneously feeling bad that (i) she's been hiding this from me, (ii) after 15 years of marriage, she's still not comfortable initiating sex, and (iii) that she's masturbating, which she still feels is dirty, but still apparently better than initiating sex.

Again, I don't really care that she's masturbating.  If she was doing it because sometimes she just needs a release or time alone with herself, totally normal and fine with me.  

But she seems to be doing it solely because she's embarrassed to ask for sex.  It's bringing back some frustrations that I had earlier in our marriage about her passivity.  So we're both feeling bad.  I would like to be able to move past this (or better yet grow from this), and I guess I'm looking for suggestions on how to discuss this with her.  I don't want her to feel bad about masturbating but I simultaneously need to deal with my emotions about her passivity in the bedroom. 

All advice welcome.