First tell your wife that you want to work on this and you hope she does too.
See if you can actually figure out what you want out of your marriage. It sounds like you were content but it can be better than that. It was obviously not what she was doing else you would not have been discounting her feelings.
Next ask her to make a list of what she wants from you. Often a big problem is that she give you what she wants in return and you give her what you want in return. Unfortunately that won't work unless your lucky and have the exact same wants and needs.
Both of you read this The Love Bank
. It's very helpful especially if she's given up and thinks she can't get that in-love feeling back. Trust me you both can.
Stating the obvious here but the MIL does need to be out of the picture. Just realize it's not the MIL's fault that she's there. Your wife HAS to address this. If you want her to address this then you have to make sure she knows you truly want the MIL away for good reasons (So you both can work on a healthy marriage).
Also mentioned somewhere up there. Don't fall into the trap of making yourself look emotionally weak. Try to be very blunt and to the point about what you want and your expectations of your self and of her. There is room for disagreement but you should have some solid boundaries that are not negotiable for you or her. I'm sure you do but make sure it's apparent that you do.
No begging as stated before. It's got to come across as desire and not emotional weakness. For example, she should know that you think being without her will make you sad. You should not say please don't leave me.
Admit that you have a lot to work on. Tell her she does too if you think it's true. It usually is. And that you think you guys and make your marriage many times better than it was before.
Again I'll say it again. Have a strong appearance because you need for her to respect you and what your' trying to do. If you swallow anything that makes her respect you less then your problems will get worse fast.