Both are correct. It's true that generally speaking we use 'helpful to' when describing an action that one person does and another receives the benefit of. For example: "The waiter was helpful to me because he explained everything on the menu." And generally speaking we use 'helpful for' when it is a thing that is helpful. For example: "It was helpful for me that the menu was written in English." But these are very general guidelines and you will find lots of examples of people using this in different ways, and both are acceptable.

You can just say 'it's helpful', but if you want to add more, for me is best.

I think is (it’s very helpful for me ) But I’m not so sure