Teddy Pasternak at 2011-08-02 09:34:30:
Oh, you're asking the big question, Scott. In her mind it's not ironic. It's simply what she believes; she'll be somewhere else, presumably heaven, and she will miss her son when she's there. And since she starts by saying quite bluntly “I'm dying, Forrest.” she is not treating him like a child. If this was for Forrest's benefit, to make it easier on him, she would have expressed herself differently.
By saying “I was destined to be your mama. I did the best that I could.” “You have to do the best with what God gave you.” she's expressing a few things; it must have been a challenge raising Forrest, but she's proud of how she did it and how he turned out in spite of him being “different.” She's also saying that in someone else's hands, he might not have turned out so good.
It's interesting how she somewhat contradicts herself by first saying “I was destined to be your mama.” and then she says “I happen to believe you make your own destiny.”
And with Forrest's concern about the future “What's my destiny, Mama?” comes the life lesson “You're gonna have to figure that out for yourself. Life is a box of chocolates, Forrest. You never know what you're gonna get.”
That applies to Forrest as in 'You're on your own now, I won't be here to help you.', but she's also talking about her own experience, how she was given Forrest and did what she had to do to make sure he had a good upbringing.
This is the moment when a mother lets go of her child. You're a parent until you die. Even though Forrest has been on his own for years and lived an adventurous life he has always had his mother there to support him. Now she lets him go, and she's at peace with it.