Being lied to and missing it - it happens.

Outgrowing my partner. Succeeding. Changing them negatively inadvertantly.

I used to be scared of infidelity but now I realise that's an aspect of a lack of self esteem and going on about it is a self actualising prophecy as it leads to you giving them attributions that they will act apon.
My best way to defend against this is to trust them and tell them they are reliable and it's nice that they only love you, out of everyone in the world. Turn up your own attraction magnet, work out at the gym, learn some cocky/funny material and bust on them then make some romantic dates and give them regular orgasms.

On my behalf I do not fear it anymore after seeing my partner snog a friend of hers infront of me, getting madly jealous and sick when she went dinner/movies with male friends after flirty phone calls.
People go on their journey. If they screw around and disrespect you, tell them about it and leave but don't think about it in circles, grow through it.