GoingMad I am so very sorry for the loss of your baby, the breakdown of your marriage and now for this burden which you are carrying.
I will tell you what I think and you can feel free to ignore me if you wish.
Tell him.
I would either write a letter or an email or I would suggest a quick coffee or whatever and set a time.
I would say it in the most blunt and all in one way you can because to go into lots of details will mess with his head a little potentially but the truth is best.
Once he knows the basic facts, then you can go into details.
I say this because a friend once wrote to me to say someone we both knew well had died. She went all round the houses about his illness and it was worse when I got to the end of the letter and he had died!
So I might say, something like, "I am sorry I never told you, but our one night together resulted in my getting pregnant, I did not tell you because I was worried it would spoil your college life and so I kept it secret, My husband and I got back together and then the baby was stillborn.
Then (if you feel this way) I would way I am sorry for not telling you about the pregnancy or death of the baby. It was all very hard and painful and I could not face talking about it all. I was worried I would hurt you/upset you (edit this to whatever you really want to say, but this is how I would say it, just the facts as best you can).
I would let him ask questions and I would offer to show him scan photos or any other things (whatever things you have kept from that time) and tell him where she is buried or her ashes scattered in a crematorium garden I would offer to go there with him if you can bear to do that.
You said "... it's hurting me to keep the secret but I think it's likely to hurt him by telling him . I don't know if it's fair to tell him to try and make me feel better."
You are not telling him to make you feel better, you could tell him as he has a right to know (IMHO) and then, quite aside from his rights to know, you may feel better for getting the truth out in the open.
You have not done anything wrong, I think you have tried to cope with an unplanned situation in the best way you felt able.
I think you did this as you felt it was best for your daughter and then when she died you kept it from him maybe so as not to hurt him, or so as not to hurt your dh.
I think all your actions, as far as I can see, have been to help others.
You must now help yourself and relieve yourself of this massive burden. Better to tell him now than to rock up at his home in 40 years time with this situation.
Please also look for some counselling for yourself as you have been through so much.
YelloDraw your comment is ludicrous.