He had sex with you, and then today has has messaged these things to his girlfriend. I guess the sex was consensual, but there are undertones here of sexual abuse and rape. Nothing illegal. But this dude is seriously abusing you. Having sex with you under false pretenses. More than a dick move. It's an abusive move. Guessing his girlfriend wouldn't love it either...seems like he left the sex part out of his messages.
I'm sorry. I find it so crushing to read all of this. I'm so sorry that you're in this cycle.
You've struggled to follow the advice here (*so* understandable; we all struggled), and you've regretted it. Somewhere inside you is a place that understands that if you just do the things we're advising you to do, things will go better for you and you'll feel better.
So please, please, please follow this advice: do not let this piece of shit take you to the airport. You've been degraded enough. Don't let him degrade you any further. You will regret every single moment you spend with him on your way to, or inside of, that airport.
Whatever 180 mistakes you make in the next 24 hours, don't let him take you. Honestly, if you can, leave tonight. Stay in the nearest hotel to the airport. Leave right now if you can, before he gets back from work.
If you haven't already, screenshot every single one of the messages he's sent to his girlfriend. Email them to yourself and save them on your flash drive.
If I had my way, this is what you'd do:
1) Print out every single message you can between your husband and his girlfriend since you discovered his affair.
2) Put them in a stack on your kitchen table, with today's right at the top (the one where he explains he's taking you to the airport, then meeting her for lunch), leaving no other note or explanation. (aside: what a fucking douche bag...he's planning to meet her directly after dropping you off. Directly after.)
3) Get your bags and a cab, leave your apartment. Leave no note, send no text.
4) Go to a hotel and pay for the room with cash
5) Get yourself an amazing dinner
6) Respond to 0% of your husband's bullshit manipulative texts and phone calls
7) Get to the airport as early as possible. Do not let him "surprise" you at the airport. Seriously, get there 6 hours early if you can. Get through security so that he can't follow you (I don't know how German airports work, actually)
8) Get home. Take a breath. Start your new life.
The leaving behind the letters isn't necessary and may actually be ill-advised -- others may have a different perspective -- but man would I like to see that boss move happen.