suggestions:

Thank you so much for meeting with me yesterday on such short notice. I really enjoyed meeting with you and learning more about you and the center. I was faced with a very difficult decision.

Unfortunately, I have decided to take the research assistant position with the Center for Social Development and Education. I want you to know what a difficult decision this was for me to make. What the Center does is very important to me, but, I have decided to take the research assistant position because it is in my field of study and will give me hands-on experience that will likely help me in of the opportunity it gives me to advance my education.

I know I would have enjoyed working with you and Donna, and I wish you much luck in finding someone. I hope that we can possibly work together in the future.

Thank you again!

I'd cut the "Unfortunately" at the beginning of the second paragraph. I don't know why, but I would.

I would also cut the "But" from

But, I decided to take the research assistant position because it is in my field of study and will give me hands-on experience that will likely help me in my education.

...at the end of the second paragraph.

It's a good letter though, vw. Just the right tone.

cereal...

I like amy's edit.

On second thought, I agree with Cindy about the "Unfortunately"

dueling "what she said"s!

Yeah, I was wishy-washy about the "Unfortunately," so I'll take that out.

Thanks guys!

some calm at the job ma would nice for today.

Hee. All together, now.

Lots of calm for askye.

Lots of calm job~ma for askye.

Calm, cool, collected job~ma to you, askye.