It sounds like there are a number of issues in your post (i.e. the lack of sex, paranoia of her leaving, messages to the ex). All of these are seperate issues, for the purposes of solving one issue at a time you should probably focus on one issue and go with it for the time being (otherwise this is going to be very messy). If you are unsure, ask yourself if the problem you are trying to resolve is rare sex life or if it is something else?
Rare sex can come about for many different reasons. Incompatible sex drives, Religious reasons, lack of or inapparopriate intimacy, grudges, poor performance in the bedroom, staleness of relationship, depression, yes even promiscous behaviour that you pointed out in your post (though I would investigate other potential reasons first). For the purposes on uncomplicating the issue, can you please give more information on when you first noticed this got started? (was it abrupt from sex every day to once in 40 days or slow transition) What was happening in your joint lives at the time? What you think may have contributed? Can you see relevance of any above listed reasons to this?
Don't cry, don't regret and don't blame
Weak find the whip, willing find freedom
Towards the sun, carry your name
In warm hands you are given
Ask the wind for the way
Uncertainty's gone, your path will unravel
Accept all as it is and do not blame
God or the Devil
~Born to Live - Mavrik~