Sorry Bigmouse, I was not able to come back here again yesterday. Now I have put together something for you based on your sketch. Hopefully it would help.
Dear Mr....
I am reluctant,but I feel like I have to write you an email in regards to my previous involvement in Mr....'s research undertakings. For the past 2 years, I have been happily and productively working with Mr... on the several of his researh projects. Mr ... is a nice and easy going research fellow. It has been a great honor seeing my name appearing on those research papers together with his and other research colleagues who have got good names in their own areas. I really appreciate the enlightment and insight I have acquired along the way, however the parts I did were not really the core field on which I like very much focus my researh or where my 100% interest is. Honestly, it is felt a bit off the area where I have been keen to explore into. These days with a few thing have come in line for me, I need to commit myself more to my own research projects, therefore I afraid I would not able to set aside extra time to assist Mr...., which is very unfortunate to say, but I have to. I hope you and Mr ... both understand. I wish someday Mr ... 's research field will cross with mine, and then we will have the opportunity working closely once again.
sincerely yours
