help please - anyone

what am i doing wrong? my 20 week old goes down for bed anywhere between 630 and 730 but wont stay asleep more than 30-40 mins then wont go back down to sleep until 9-930pm. he gets a good 4 hours during the day stretched over 3 naps. he eats great and sleeps all night waking only at 7am to eat. my only issue is getting him to sleep and staying asleep without whinging for 2 hours until 9-930. iv spoken to a sleep consultant who told me he was tired. but he sleeps great i put him down just as i see the first tired sign etc. he just takes a while to settle. he also hates the dummy and swaddles and is teething again (he cut bottom teeth at 4 month exactly) i feel like a failure everyone else is doing the whole mum thing with no issues and im here struggling like a idiot. i just feel really down about it and everything in general.