"even going to work feels like a holiday."
I remember that feeling well! Adult company and conversation, being able to get things done and dusted not just kept on top of, sense of achievement not just being on the relentless treadmill - I was so happy being at work then!
First thing - cut yourself some slack.
Consider Rule 1 of First Aid - the first aider has to attend to their own safety before aiding another. Similarly, the Safety Instructions on an airplane - adults must put their own oxygen mask on before putting the masks on their children. These both spring from the same reality - not taking care of yourself first reduces your usefulness to others. In first aid, rushing to someone's aid before checking the scene could have you electrocuted/gassed as well, giving the next person two people to aid. Putting your children's mask on first could have you passing out from lack of oxygen, leaving your child breathing but panicking and alone.
So, before you attend to your child's needs, you need to ensure there is a functional adult available to attend to your child's needs. That means taking care of yourself FIRST. Only then are you able to turn your attention to your son. It isn't selfish to do so.
Next up, your son's moaning and whining and tantrums. Dear God, it's wearing!
What worked for me was listening to the moan and deciding whether he had a point or not. If he had a point (e.g. his feet actually hurt or he was actually hungry) I'd address that. If he was just moaning through boredom or habit (e.g. I don't want to be here) he got short shrift and basically told to suck it up. Consistency is your friend. Whining for whining's sake never got him anything.
Whining, as in whiny voice - 'I can't hear your words when you speak in that voice, you need to speak in your normal voice'. Consistently.
Endless questions - if the same questions are repeated; answer once, answer twice adding that you've already answered, third time don't answer but say there's no point in asking a question if he doesn't listen to the answer, you've told him the answer twice already. If it's endless different questions; you're asking for asking's sake, I want you to decide what you want to know most and ask only that.
You are doing fantastically well, please believe that. At 3.5, you are in the absolute trenches right now and the sheer relentlessness makes it very hard to keep going. But it will get better, it really will. But in the meantime there are a few things I think you should consider doing for yourself. Remember, you're more use to your son as a well person than as a drained person, so stop any thoughts that looking after yourself is somehow selfish - it really isn't.
You said you "collect him early from nursery wherever possible". Well you can knock that on the head for starters! Instead, go for a coffee, sit at the window and watch the world go by. Or read a book, do a crossword. Or take a walk through the park, sit on a bench and admire the flowers. Treat it as some decompression time for yourself. Picking him up early is not the priority, you are.
Similarly, consider taking a day's holiday now and then for yourself - just for you. Do something that's just for you. Take your son to nursery and pick him up at full time, not early. Do something, do nothing - just prioritise yourself. Remember the Safety Instructions - you need to put your oxygen mask on first. And you need to take care of yourself first.