I don't think so/I don't think there is

A: But there must be something we can do.

B: I don't think so / I don't think there is, unfortunately. If the love is no longer there, what can I do? What can she do? At least I had two good years with her.

Which one of "I don't think so, unfortunately" and "I don't think there is, unfortunately" is the better choice here according to you? Thanks.