Correct me if I am wrong but you read a text message that said sorry my snoring woke you up? You might not have proof of a physical affair but how does one end up next to a person close enough that he/she hear them snoring?
Based on what you've told us I am inclined to believe there has been a PA. There is definitely an EA going on. She spent the night at his house. Your partner spent the night with another man.
By chance did you print out copies of the text messages you saw? The phone log? That will come in handy
When confronted most WS's start with denial such as I don't know what you're talking about, there's nothing going on he's just a friend, why are you so suspicious, why don't you trust me. But when confronted with evidence of infidelity they sometimes switch to minimization such as we were only flirting it's no big deal, nothing happened and nothing ever would have, you are overreacting.
If that doesn't work they sometimes switch to DARVO and try to put you on the defensive. You don't spend enough time with me, you work too much, you don't support me emotionally, etc. This is yet another tactic to deflect guilt and hopefully convince you that you are in the wrong and hope you let it go AKA rug sweep
I suggest you sit down with her, look her in the eyes and ask is our relationship okay and see how she responds. If she looks at you and says everything is okay, why are you asking, you now know she is willing to lie to your face
If she responds with why are you asking tell her to just answer the question. If she says everything is fine then ask her is there anything I need to know about and if she says no Then you ask her are you interacting with any other guys in a manner that is harmful to our relationship. At this point she's probably going to wonder if you know and what you know and this is where the minimization, deflection, DARVO, etc start to happen.
At this point if she has not confessed then you simply say I know about him and see how she reacts. Do not tell her everything you know, let her wonder what you do know. Right now you only know what you have discovered and chances are very good there is a lot more that she probably will not tell you. You may get some details here and there AKA trickle truth
When confronted if she does not break down begging for forgiveness or if she just dismisses you then it might be time to end the relationship. She has to think you are willing to end the relationship if she does not come clean but if she thinks you will just get over it she will stick to her guns and just wait for you to get over it but it will sit in the background festering until one day it erupts and by this point the relationship is in serious trouble
You are in for a rough ride. Your mind will probably be processing a million thoughts a minute. Your emotions will make you feel like you are on a roller coaster. Discovering infidelity is an incredibly traumatic experience. It rattles you to the soul because everything you thought was real and safe and secure just evaporated
As others have suggested, schedule a full STD panel. Do not believe her when she says nothing physical happened because she has no reason to tell you at this point if something did. At this point you need to prioritize your mental and physical health.
When I confronted my wife thankfully I had the frame of mind to record the hour-long conversation because I knew there was no way I would be able to remember everything that was said and how it was said and I knew I would need to revisit our conversation many times in the future hence the reason I recorded it. There were a few instances during counseling where my wife said something to the effect of I never said that or I didn't act like that or you said this and I was able to say you are wrong and if you want we can listen to the conversation
I'm sorry you are part of the club that no one wants to be a member of but you are in a great place for advice and support. Most of the members are from the Betrayed side but we do have a handful of people who were the cheaters so they are able to offer a counter perspective on things
Stay strong, don't get angry as that could be used against you, put yourself first. The only way reconciliation has a chance is if the WS admits 100% fault and is willing to drag his or her bare butt through miles of broken glass if that's what is needed to repair the relationship. I would strongly advise against seeing a MC, find an IC that specializes in trauma because believe it or not you are going through an incredibly traumatic experience.
Once you feel like your feet are back on the ground and you can't think clearly and if you still feel you want to work on repairing the relationship then consider MC. I would also suggest you demand that she enters AA and starts seeing an IC on at least a biweekly if not weekly basis. I told my wife that until I believe that whatever was missing or broken inside her has been found and or repaired I will not consider her to be a safe partner
Good luck and we are here for you