A trip cancellation letter is basically to inform the person, with whom you made some commitments, about your revised decision. It happens when an individual or organisation feels that they have made a wrong decision which needs to be corrected.
It shows that the writer is not comfortable with the step she/he took and now wants to make amends. However, make sure that your decision is final before you write this cancellation letter. It should be written well in advance to give enough time to other party.
In the opening paragraph, give complete details of the previous plan and inform your addressee about your revised decision. You should give a solid reason that forced you to change your mind.
Then in the next paragraph, you should make the other party realise that you feel sorry for this unexpected trouble and that is why you are writing them well in advance. You can also suggest some remedy to help the other party in this awkward situation.
Make an apology again and end your letter with some professional and courteous sentences followed by your name.