My kid didn't sleep well at night till she was three and a half. "Sleep training" just doesn't work with some kids. 18 months is still very little. Do what you can to get through. Is there really no way your husband can get himself home from work, though? That second nap in the car is not helping.
Clean house, schmean house. Germs are good for them. Show homes aren't happy homes.
If you're worried about ignoring her, then just change it- have a rule such as 'no computer before 1pm' and just get out a huge pile of storybooks, toys, playdough, whatever and make the effort to chat to the little one more. Nursery rhymes are great. You couldwatch something naff like The Wiggles and learn the songs with her- little kids love seeing their mums singing alone to their top choons. But go easy on yourself. I understand how seductive the internet is, especially when you're lonely. You do feel like shit when you realise you've wasted three hours on it though. THAT is fixable- just limit yourself; you may find it stops being so attractive, oddly enough, the less you use it.
One more thing. It may sound stupid, but doing home exercise vids helped me with depression. My kid loved joining in and I felt happier, more energetic and more able to tackle things.
Overall, you sound like a kind, loving mum. Not useless at all. Be nice to yourself and try to enjoy this time with your little one. 18 months is such an interesting age.