I used to do stand up (wayyyy back!) with moderate success. Lukas it is unfortunate that your audience were all Australian. I guess this might have been avoided if you could have either been part of the audience for a bit or heard other acts on before you.
Don't be so hard on yourself i.e. it was a first gig - did you expect to be on a roll (so soon, already?) Being nervous is a good sign, in moderation (I had occasions when I didn't care whether they laughed or not...indifference is not good!) Have you thought about going to a comedy workshop (if they exist somewhere near you)? You will learn a lot from that.
I will pass on a technique that worked for me i.e. to relax me just when I was about to go on stage....I thought of something that mellowed me/made me feelgood.
Re. mumbling - make a conscious effort to s-l-o-w down (to counteract sounding nervous) and try and to pronounce words clearly i.e. in particular, the punchline is key!
Re. having ADHD - is it a problem when performing? If so, it's wise to let them know you have this then make a joke about it. Maybe you can say something like....I have ADHD....luckily I don't also have OCD as well, else I'd be keeping everything neat and tidy....but not for very long.
Have a plan B if everything goes out of your head i.e. either improvise (maybe something catches your eye about the audience) or, maybe, have a prop to do something interesting with.
Have some prepared responses to anticipated heckles - being one step ahead and witty with it is a huge plus!
Try and enjoy the experience - it's just a laugh after all and especially if you're not yet getting paid!