She told me she was out for a meal with him and I don't even know why I wanted to go or what I thought I would be able to achieve. I saw them through the window and walked away.
You're stalking a woman who has ended her relationship with you. It's not up to you to say you don't accept her ending it as you deem her reason invalid.
I just can't cut her off I love her too much
You can, but you don't want to.
You need to reframe this in your mind. This isn't a romantic thing. She's ended it. You need to block her, heal and move on.
And if her husband is as abusive as you say, and you truly care about her, then you won't continue to speak to her and follow her places because if he finds out then it's her and the kids he will punish with further abuse.
This is madness all round.
You do have control of your response to the situation, it's just a situation you wish wasn't happening. But it is.
Cut contact completely and block everywhere. Currently you're making life harder for yourself and contributing to the risk of her husband being abusive if he finds out you're talking and she's telling you places they are going together. He'll punish her if he's an abuser and finds out about that.
If you love her, let her go. Yes, she's making a self destructive decision to go back to her abuser. But you are only heightening the risk of that abuse.