Of course a sad look can also just be a badly caught moment, but sometimes it can be due to some underlying reason of stress - can be many things. Sometimes there is stress in the herd without we notice it - some bully making trouble for those who are not that strong mentally - I think it is extremely important to match the horses which are on pasture together, because we don't give them the possibility to choose for themselves whom to be with.
It can also be a working issue (I don't say it is any of these things with your horse - please don't misunderstand - I just want to give you ideas to look deeper at her so to speak :)) - I am a keen follower of Klaus Hempflings principles and he says that a horse should never do hard work for longer periods without pauses in between - and hard work is for example lunging, because moving on a small circle is hard for both muscles, joints and bones. He advices never to exceed 10-15 minutes of real lunging without making a pause of at least the same length of time. You can do it many times a day, but the horse needs pauses.
The usual dressage/jumping training where the horses are worked in trot and canter for hours is very bad for them - that is also the reason why for a dressage/jumping horse the average age of death is very low - they are finished when they are 10!!! In contrast to this in former times the classical way of training a horse startet earliest in the age 4-5 years, when the horse was fully grown, and the horse was not considered educated until it was perhaps 10-12 years old - and then it would live a long life as a well trained riding horse (25-30 years) - those times are long gone unfortunately :(.
There could also be an energy issue - for example to little energy in the food she is fed - it is very important to horses to have oats - even little fat ponies should have just a tiny amount every day, because it gives them a good energy.
Most horses like it very much to have their own stall in the night time, or at least to share the night space only with horses they feel really comfortable with - in open stables there is often some underlying stress which is unnoticed.
What also could be the case with you mare us that she still has the previous abuse inside her and that she could be a bit sad becaue of that still - but that will disappear for sure with the right handling and you certainly try to do your best.
Anyway - with all you have explained, I still feel it would not be so bad to slow down things a little - just to make sure there is no pressure.And you will fell it if she misses the work, because then she will show you that she wants to do thing. On the other hand if she just enjoys simply being calmly with you she might also need just that sometimes in between - you could take some walks, or just sit by her - whatever - it does not mean you cannot do things with her - just adjust the workload a bit.
Whish you good luck with your beauty - looking forward to seing some more pictures of her!
All the best to both of you...