Welcome Starmoth.( I'm trying to imagine how it feels right now on the southeast coast of England as the temperature here climbs toward 100 degrees...)
Strictly speaking, "hardening off" is the process of gradually acclimating a plant grown indoors to outside conditions to reduce transition shock. This is usually done with seedlings started in a warm environment such as a greenhouse before they are set outside in lower temperatures in the spring.
That said, there are other reasons why you might need to acclimate or protect a new planting. Yesterday I finally potted three little zinnias that had been left behind in a 6-pack. They had been in the shade, so I put a little shade dome on the pot to keep them from wilting in the heat (probably not necessary, but it was an easy precaution). I might leave that on for a couple of days to reduce heat stress, but normally I don't harden off nursery plants. I do protect baby plants if it is frosty, or shade them if I think the sun is too intense while they are establishing their root systems.
I can't imagine that you need to harden off at this time of year. Some additional information would help: What are you planting, how big are they, and where were they grown? Are you going to plant in pots or in the ground? Are your nights still dipping down close to freezing? In mild weather, sometimes just setting new plants in a protected spot in dappled shade in the garden for a few days is adequate acclimation.
It sounds like you might have jumped into the deep end of gardening prematurely (pardon the mixed metaphor) and that can be more frustrating than fun. I hope you can find a comfortable level of commitment and it should definitely NOT involve hurting your body and stressing you out. This I can tell you: I felt wonderful the first time I blended a tasty and effective tea entirely from herbs I'd grown in my garden!
