>That day my world shattered

>We already planned a billiard night the next day and I didn't want to cancel on short notice

>We met again the day after because we had already bought tickets for the spa

Dude, wtf. Your world was shattered, but you kept meeting up with him repeatedly after?

>He mistook that as an invitation and told us he'd be happy to accompany us

Yeah, he didn't mistake anything. The best reply was "No, that's alright. We want some alone time :)". If your friend can't accept that you and your girlfriend need time alone, you need to seriously reconsider that friendship.

>J told me she was too tired to search for bed sheets and as her dad wouldn't allow someone to sleep on their couch, we should just sleep in one bed.

And that sounded reasonable to you? That you were doing it because her dad would prefer two men sleeping in her bed, one of which was nearly a stranger who was making unwanted physical contact towards her, to someone sleeping on the couch? And that she felt this was the right thing to do? And that in that sleeping arrangement she was sleeping in the middle between you guys?

>I did what everyone in that situation would do and kindly told him to fuck the hell off.

That is absolutely not what everyone in that situation would do.

>We decided to still take my best friend for the trip to one of the tourist attractions

Dude WHAT. Your friend has physical sexual contact with your girl, in your presence, you have a mental breakdown over it and argue through the night, and the next day you still bring him along? I'm struggling to imagine anyone doing that, honestly. The only plausible explanation I have is that you were paying for things and that's why they needed you there. It definitely sounds like you're the driver for your friend and his girl, not the other way around.

The next part was pretty damn dark, but I think you really have to understand that you were being absolutely blind to all the warning signs.

Stay in therapy, and don't say things "Don't trust anyone", because that wasn't the fuckup. The fuckup was burying your head in the sand and ignoring every red flag along the road until you fell off the damn cliff.