Im suing my business partner for breaching our partnership agreement, and before I sue I want to see if I can settle with him by sending him this letter. In this letter I want to state my case to him and explain why we can no longer work together and my side of the story. Can he use this letter against me in court. If so what should I add on the letter that it cannot be used as evidence.
Yes. It is evidence of your statements. It may be an "admission". The only way it cannot be used is if it is part of settlement negotiations, and safer if you state that in your letter (e.g. the following is for settlement purposes only).
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
Construction / Development Lawyer
Yes, anything you put in writing could be used in a later lawsuit. Settlement discussions are generally inadmissible. Include a statement at the end of the letter that the letter is an attempt to resolve a dispute, or is an offer to compromise, and should not be construed as an admission of any fault, liability, or other wrongful conduct of any kind.
Partnership disputes can be very complicated. Before doing anything or sending anything, you should consult with a lawyer in your area.