I bf my ds for 17 months, he stopped only 1 week before DD came alone. I really did not want to BF dd as Ds, from about 4 months old, would only BF lying down, so was a pain to get out anywhere for any decent length. I was the same as you, planned on stopping at 6 months, but he would not take any sort of bottle, and was very sick at 6 and a half months, just after he had started solids, so he did not touch food again till he was nearly 14 months old, so I just kept on BFing him.
I said to Dh, I was going to bottle feed dd, as I could not face it again, even though DS had been easy to bf, never took very long, never had any issues bfing, was just the lying down thing which meant I could never really go out, and I never ever felt comfortable Bfing in public.
But dd came along, and I gave it a try, more because I had with DS and it was expected than anything else, she had a tongue tie, and was on and off 23 hours a day, and that could have easily been the time to stop, especially with all the midwives and HV saying to try her with a bottle, but I got stubborn about it, and had to fight to get her seen by the specialist at the hospital, and have her tongue tie snipped, and after that everything went fine...and she bf till 18 months.
The point is, you do not know how you will feel till the baby is here. But, if I had decided to go onto formula with dd, after trying to BF, I would not have agonised over it or felt guilty. I BF dd purely out of stubbornness because there was an issue.....DD is exactly like me, contrary and stubborn, if you tell her the sky is blue, she will argue it isn't, just because she can!