You are worrying to much about her. You need to focus on yourself. You can't control her or what she does. You do have to decide for yourself if infidelity is a deal killer for you. If it is and she does cheat, then you know how you will move forward. But right now, you can't worry about that because you have NO control over it. You shouldn't be thinking about filing for D just because you think she will cheat. Filing for D now will probably push her in that direction. You need to focus on your DB'ing instead of what she may or may not do.
Me: 48 y/o W: 47 y/o Together: > 20 yrs BD: Dec '15, then S 2nd BD: Mar '16, then I filed for D April '16: started piecing