I had the same sort of problem wth my little boy in that he wouldn't nap and would get so overtired and grouchy that I just didn't know what to do. In the end before he got too tired I took him to his nursery, changing his bum, giving him a massage, then swaddling him and closing the curtains in his nursery - though I didn't put the blackout blind down, so it is dim but not dark - and either giving him a bottle (I know he shouldn't have a bottle immediately before sleep but it got us sorted!) or if I knew he wasn't hungry singing to him to wind him down, and now we use the shush pat method. Sometimes he goes to sleep now within a minute or two of shush patting.
I have read differing views on naps that the baby should be downstairs during the day, hearing noises etc. so they learn night from day but my little one really can't sleep downstairs. He sleeps for 10 mins and wakes up so before long he is so overtired he is beside himself. Plus like your baby, he has always slept fairly well on a night so I wasn't worried he would get confused.
If your little one is waking up after 20 mins could you go to him just before 20 mins and sooth him back to sleep before he wakes up properly. If you did this a few times maybe he would start to sleep through the 20 min mark?
Hope this helps, or if it doesn't just wanted you to know I'm thinking of you
Sally
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