My DS is a similar age (21 months) and I also went back full time when he was 9 months old.
I am a teacher in a College so I'm very fortunate to have the school holidays, but this is my third week back after summer and I'm having a down day 
My ds loves his nursery and does so much there but I do feel guilty that it's not me doing all of those things.
To make matters worse, he is usually tired after nursery, so I feel like during the week I get the worst time of day with him because he's grumpy. He does enjoy his bath and bedtime stories though so we make this special time each evening.
I've no advice really - I think most (if not all) people feel as we do and the guilt is part of being a parent.
I just try my best to make the weekends nice and try to do things with DS during that time e.g. park/library/swimming/day out etc.
I shop online so we don't have to do that and I do housework when he's in bed at night - it means I don't sit down much in the evening or have much 'me' time but it's a compromise so that I can devote Sat/Sun to DS.
Being a teacher means I often have to do marking etc. at home but again, I do it when DS is sleeping. Although, I am now much better at using the few free periods I have at work to do marking etc. so that I have less to do at home.