Whilst I agree that marriages already struggling would have found it particularly hard, I don't think finding it hard means your marriage is struggling.
It's just not normal to spend 24/7 with the same person in the same space, however much you love each other. DH retired just over a year ago and, whilst life is good, it took some getting used to being together all the time. In the 40yrs we have been married lockdown was the most time we have spent together in the same space, even counting retirement.
Generally We both have hobbies, friends and individual stuff that we do both in and out of the house. He's a birdwatcher/photographer so spends long days out and I am most definitely someone who likes my own company. So it's been challenging, not bad, not stressful really but definitely challenging. And I've loved having my boys home from uni, they are both fab but, again, it's not normal for teens to be home so much.
Most couples with at least one out at work would spend around 75hrs a week together and that's increased to 168hrs a week for many. Add children and wfh into the mix and even the most placid person would start to feel hemmed in. We are lucky, we have plenty of space to get away from each other when necessary but obviously that's not the case for all. Then you have introverts, extroverts, usual differences in behaviour patterns, optimists, pessimists, night owls, early birds etc etc. So just because you are finding it difficult doesn't mean your marriage is going sour.