I am afraid to come across as demanding and possibly burn a bridge that I may need in the future. At first I was worried that I had given this prof too little time (6 weeks, but I received a response after 2 weeks) and my request had an apologetic tone-- I prefaced my request by saying, " I know you must be busy with your current students and research..." and ended with an " if you are unable to do this I understand" as a way to be polite and undemanding; now I think I just gave an easy 'out'.... the prof's response was: "I can't provide you with a letter at this time. Good luck." I responded with a "thank you for your time and thanks again for all your help in the past." I haven't seen this person for 3 years so I can't think of something I did that could have damaged our relationship. Either way, I feel like it may be better to move on rather than dwell on it, but I don't know how (or if) there is a way to do damage control for this.