I think you should send the letter. You answered it yourself really, with these two comments "I have done the pro's and con's and the pro's truly outweigh the con's" and "I think the problem is I want to know for sure he would consider taking me on again before I send the letter." Here you are saying you've thought about it and it is the right thing to do, but you are afraid you might be rejected. That is certainly reasonable! In your shoes everyone would have that fear. In the worst case, if he does reject you, at least you tried and you can stop obsessing. He did keep the possibly open right? Why did he terminate you? If he does reject you then what can you do, it is nothing to be embarrassed or ashamed of, it would sting I'm sure but in the long run be better than obsessing over the decision for weeks and then finding out.