Feel like my world is falling apart

Don’t know what to do…

for a few months I’ve noticed my wife acting differently towards me and being dishonest with what she is doing and where she goes.
I’ve approached her about this and been told every time “don’t be silly it’s all in my head”

I just knew something was up so I checked my phone bill and sure enough when she claims to b walking the dog or shopping etc there were hour long calls to the same number

so I call the number find out who it is and stupidly looked at her phone messages and straight away seen 1 about from the other guy saying some very private things about a certain part of her body.

I asked her about it and was told he’s just an old friend turns out old boyfriend and said was just remembering old times but that’s not how I read it.

I now have been in a constant battle where she is on the attack towards me say it’s my fault and I pushed her towards it.

thing is we were actually getting along real well apart from that little nagging feeling I had.

I now don’t know what to believe that she tells me.

is there more to it?
really just friends?
how can I trust what she tells me ?

im feeling lost and can’t talk to anyone so thought I’d give here a go

thanks