Op, your concern is real and very common in relationships, it gets worse when guys have dated for long periods and there is not much excitement anymore, when this happens the less mindful partner (which is your woman, according to your narrative) moves on to the next interesting thing while the more mindful one (you in this case) starts feeling ignored.
It doesn't mean she don't love u anymore (at least not yet) but truth is there is a great danger in a married woman or man ( or even people in serious relationships) finding excitement in any other thing than her partner (in this case, it's her phone), it leaves the other partner downcast and may eventually lead to a downward spiral in d relationship.
You need to communicate your concern to your partner and avoid assumptions, that's always the first and best step. Have a serious conversation and let it be clear that it's a serious conversation. U don't need to threaten to leave her, that's not necessary cos it might just be a temporary excitement but your happiness should be her primary concern and vice Versa, and so, bring it to her notice and if you are still important to her she will apologize and you will see improvements in your relationship.
The challenge now comes in when u don't see any positive changes from her, if you don't, bring up d discussion again after say a month, remember that some people are just naturally not mindful, you have to give her the benefit of doubt.
On your 2nd conversation bring up the conversation of mindfulness. Ask her if she understood what it meant to be mindful in a relationship (you sef go and google it before the conversation) and if she thought she was mindful enough of your relationship.
Hopefully she realizes she can be more mindful and starts practicing that in the relationship. Give it another month or two and review her improvements at the end of that period.
In between these months make sure you guys are engaged I.e. Doing fun things together, join her in those things she loves doing and encourage her to join you in things you love doing e.g. If u love watching EPL then encourage her to, if she loves watching Telemundo then watch with her, it's all in a bid to make you guys think of each other when doin the things you love. If it all goes well then u should have your woman back.
Relationships take conscious effort to succeed, it's best when both partners understand this concept. Wish you all the best.

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