Haitch, give up thinking you will ever please her, because that sounds impossible. Think of little things that please you, a bubble bath, an evening sewing, an evening going out with hubby, and, come hell or high water, do them. Mum is so, so lucky, but just csn't see it. Have you sorted out power of attorney while she is still well? Your own?
In the end, your mum - like all of us - is responsible for her own happiness. It does sound like absolutely nothing can make her happy, because she is grieving for your dad, and that is that. That might sound 'tough' but there it is. She's a widow - that's it. So am I - that's it. (And I lost my husband a lot earlier than she lost hers.)
May I ask what she was like before she was widowed, and what kind of marriage she had? I'm wondering whether she was always your dad's 'princess' and pampered and fussed over by him, and made to feel special ,etc etc etc, and that's what's she's missing? Or maybe of course she simply misses the person she loved so much.
But for whatever reason, there is nothing you can do to 'make' her happy, and that is what you have to both accept, I would say, and 'insulate' yourself from. You can 'sympathasise' with her verbally when you are with her, then walk away, and 'put her out of your mind' and get on with what you can of your life.
Speaking of which, do you have any outside help in for her? It doesn't matter if she doesn't want it (!)(so few do, sigh, and just want us to do everything!), but for your own sake, and for the sake of the rest of your family, she 'must' accept some outside help as well as yours. That's 'the deal' for her not being 'put in a home' etc etc.
But it does sound like the most important thing for you is to have a kind of 'mental insulation' from her gloom and sadness, which you can do nothing about - you can't bring your dad back, you can't make her young again, etc etc.