Which is correct grammar, "amazed at her immense capacity in love," or "amazed at her immense capacity for love?" Even then I still have trouble settling the difference. Please elaborate and please help. Thanks! :)

Capacity means it has the ability to hold something. Saying that something has a capacity for something, it means it can hold that amount or that item. If you say a capacity in something, what is the capacity in? It just doesn't make sense that way.
I hope this helps.

"Amazed at her immence capacity of love" is gramatically correct it is 'the capacity of' and not 'the capacity for'

This is a word use problem. To have a capacity FOR something (whether for love, for seeing another's point of view, for solving complex problems, etc. etc.) is correct... conversely, to have a capacity IN something is not. The first is correct, the second is not.

The second one is correct.

Neither..."Amazed at her immense capacity TO love"...

You're using "capacity" like ability...replace it with "ability" and you'll see that neither sentence is correct.

You don't have an ability "for" something. You have an ability "TO do" something.