It sounds like you have (and are probably sending) mixed messages yourself. My husband could be having an affair and I might be the last to know because I currently trust him and his electronics and what he is doing is "his" and I do what I do on my computer and phone, etc. and that's "mine". Feeling you have to violate another's privacy to feel secure/good about yourself is not a whole lot better than giving cause for someone else to think there is an issue in the first place.

I would decide what I want (more sex, less ex-) and state that to your boyfriend. I would explore for yourself why you feel less/desirable because he does not want as much sex and look at what is "really" going on with the sex and the ex-, if there's things on your part that you can do to help yourself with how you feel.

If someone else wants to lose sleep texting all night, that doesn't bother me; however, if I am personally worried my boyfriend might go back to his ex- then I need to look at my relationship and boyfriend harder, to see if they are what I actually want.

Enjoying sex and wanting more, your boyfriend should be willing to work toward giving you that if he loves you and wants you to be happy? No matter how he currently feels, even once a week instead of twice a month should be doable to him, it's not like you would be asking for nightly, with marathons on weekends :-) However, if you just feel bad about yourself because he isn't not initiating sex (what happens when you initiate?), that is more your problem than his?