Coping with puppy meltdowns?

Currently struggling.. we got a puppy a few days ago. It's our first time having a dog of our own, but we both grew up with dogs. He came from a chaotic home that didn't have time for him, by the looks of things when I picked him up, he spent his time shut in an empty room with just a bed. When he was out, there were two kids acting frantic around him. He's a 5 month old german shephard cross (not sure what the other half is). He's also very underweight, but we are working on building up his food and looking to add a calorie dense additive to it.

All that considered, he's a good and smart dog. Extremely sensitive. He can pick up on your mood and will absolutely match it. Currently walks are too much for him, all we can do without him getting overstimulated is take him in the garden, and play inside for short periods.

It's the evenings when he gets frantic. Pacing, whining, biting, accidents in the house. I've tried to keep things chill consistently, low lights in the room, not moving much. I'm neglecting my own needs trying to get him to relax – which is counterintuitive because obviously after just getting home from work and not even having a drink, im easily stressed, which he picks up on (today was my first day back at work since getting him)

Enforced naps have been the only way we can get him to chill out in the evenings, he can't nap or relax outside his crate when he gets worked up. He is absolutely fine in the crate any other time of day- but when he's overtired, closing that crate door is the end of the world for about 2 minutes until he falls asleep. I sit with him until he falls asleep, so he doesn't think he's being punished or abandoned or something. I'm honestly so terrified of doing anything wrong and think I'm going to traumatise him. I feel horrible putting him in the crate when he's having a meltdown, but its the only way he will relax. Like you know when toddlers start crying and throwing themselves on the floor when they're tired, then when they're put to bed they'll absolutely pass out? That's what he's like.

How do I deal with the evenings? There must be something I'm doing wrong if our evening routine always escalates into him having the equivalent of a toddler meltdown. I know that me getting stressed when I'm on my own with him is only making it worse, and I need to sort that out. It's hard to keep calm when I'm exhausted. Im trying. I spent my day at work today worrying about him, even though my perfectly capable partner was with him, and got him to settle in and out of the crate absolutely fine.

As much as I absolutely adore him, I am really struggling to adapt to this. I struggle with change as it is, and we have always had a very quiet, routine based house. Now we have to adapt to an entirely new routine, and I think I'm struggling as much as our puppy is. I feel guilty and I'm so scared I'm doing this all wrong, even though when he's not having a meltdown he is absolutely glued to my side. I don't want him to hate me because I'm the one dealing with him in the evenings, and I'm messing it up.