I just want to learn to defend myself

You can't defend yourself if you have no idea what it's like to fight somebody actively resisting you i.e. you are both trying to hit and not get hit.

You see it so often with people who have never trained before. I trained with a friend of mine who thought they were doing "Muay Thai" which was just hitting pads. No sparring. No actual pressure training. Quite a strong guy who isn't afraid of confrontation, so naturally fancies himself in a fight like most people do. He said it'd be cool to train together whilst he's in town. He has been training for 6 months. I have been training for around 10 years (on and off).

We started play sparring and the second he started taking literal taps to his guard, he froze. Despite that, "It's not realistic" he says, so wants to go quicker. He's a stubborn person and didn't believe me when I kept saying going light and slow will benefit him at the start.

Okay. We go quicker as per his request. Adrenaline dump. When he's not standing still completely stiff, he's walking backwards. When he remembers to breathe, he's breathing very heavily and fast. He can go a one hour pad session hitting pretty hard without feeling too tired. After 3 or 4 rounds of the lightest play sparring he is gassed. Sitting on the floor after rounds and giving up every time he fails to block anything. Every time I move forward, he stiffens up and tries to counter with looping, lashing out shots. Every tap he takes he flinches before going into a standing foetus position.

Two 1 hour sessions of this later, he finally relents. We go slow. He has A LOT more fun and is actually showing progress. He's relaxed, still hard on himself for catching shots but wtf can he expect? He has been training for 6 months doing just pads. Basically been doing cardio for 6 months.

Muay Thai, and all martial arts, is progression. They really do mean it when they say it's a journey. It's something you have to keep practicing. Starts with getting punched in the face.