Read again.
Being an adult requires that you only take as much responsibility as you realistically can. If you know in your heart that you are being sincere, but you don’t see anything from her side, let her be. It is not your job to make her happy. It is HER job to make herself happy. In the same way, it is not her who is stressing you out, it is YOU. So, let it go. But be the person who you won’t regret being. If being your ideal person means that you take responsibility for your MIL’s happiness, well then… (I hope the absurdity of that sentence is clear). You can’t be responsible for her happiness.
Even if you were angelic, there would be things about you that people wont like. If not your MIL, then someone else. So, be the person YOU would be happiest to be.
If or when she gives you clear hints that something is wrong, ask her. If you don’t get a satisfactory reply; ask your husband. If he can’t figure it out either, let her deal with her feelings and talk to you when she is ready. If you don’t play the game she might be playing, it will have to end. I know it is easier said than done.