Hi, i don't know how recently you've gone back to work but just to say that you're not alone and it does get easier the older they get and the better you get at organising your time.
I went back FT when my DS was 9 mo and i suffered a brief period of mild depression for the very reasons you've stated so you really must look after yourself and work in partnership with your DH. Make sure you make the time to talk to one another and support one another. It really is crucial.
Try to focus on the good things that happen every day and the things you do well, rather than the things you get wrong. We all get things wrong sometimes and it can be very easy to concentrate too heavily on the negative. Repeat the mantra - you are not a shit mum, nor a shit wife. You are doing your best, and by the sounds of it succeeding, to raise a much loved child in a happy environment. Your DS sounds delightful - you are the main reason!
You will have the occasional wobble. I am currently over-analysing everything my DS does, all his behaviour, and putting some of it down to me not being around enough when in fact, and in truth, it's more likely that's he just an incredibly normal toddler. In all other aspects he is happy, smiley, outgoing, sociable, loving etc etc. He's going through a stage of biting and getting very frustrated which is emotionally very draining, but it's worse when you work FT and have the accompanying guilt.
You and your DH are excellent role models and you are leaving your DS in the care of a loved and trusted family member when you are not there. Please remember that it's much harder on you than it is on your DS. He is loved and cared for and is undoubtedly secure in that knowledge. With time, you will learn the art of juggling, look back and realise you did far better than you gave yourself credit for.
Oh and the best tip someone gave me was "always have some time off in the diary - even just a day - to look forward to". I do this now and it really helps get you over the psychological humps by looking forward to spending quality time with your DS. It makes you cherish that special time all the more.
And on another plus side, when your DS becomes a toddler you will be able to go to work for a rest 