The worst result is to break up,and I'm fine with it!

One uses ‘大不了’ to suggest he/she is ready for unpleasant things to happen so no one can use the possible unpleasant result to threaten him/her. ‘一拍两散’ means an unhappy ending, no cooperation or relationship any more. Put them together, '大不了一拍两散' means the speaker is not fear of the broken relationship.

The worst ending is break up。