I'd concur with everyone else - get her on poultry grower food; with water always available nearby (I'd say clean water but ducks don't generally have that for more than 2 minutes! ). Change them at least daily - wet food is ok,mouldy food is not (my ducks and chucks prefer the food wet for some reason)
I'd also add a bit of Brewers yeast to the food for good measure. 
The grain was probably ok, but lettuce doesn't have much nutritional value, and spinach can cause problems as it is thought to bind to the calcium so it cannot be used by the duckling; so I would only feed spinach sparingly. 
If you are feeding greens then chick grit should also be offered or she won't be able to grind it up in her crop. 
This is a good read
https://www.backyardchickens.com/t/242460/the-ultimate-list-of-duck-treats-and-supplements
On top of her diet, if she is all alone then she can get stressed which won't help. 
I took a rescue in last year and it has turned my life up side down, thankfully I have enjoyed every minute of it! But without having the support of my partner I couldn't have done it without taking at least 4 weeks off work! 
The poor thing got far too distraught when left alone, so had virtually round the clock care and attention. 
If you are having to leave your duckling alone for periods of time then she will be getting upset, not eating, all the while burning more calories calling/looking for you...
That's not meant as a guilt trip in any way! Taking in a lone rescue duckling is a bigger commitment than most people realise - 2 or more at the same time is soooo much easier to deal with 

If your long term plan is to keep her then it would be worthwhile getting her a friend or two (I wound up getting my rescue 12 friends but you don't need to go mental!)
Teddy bears or a mirror help with very young lone ducklings but I can't see her being fooled at 7 weeks! 
Keep us posted as we're all rooting for her