You shouldnt feel guilty, you are doing a great job by even trying.. Like you said, nature sometimes works different as we have planned.. You shouldnt feel like you let them down, just be happy you have formula to add ..
In the beginning I didnt have enough milk pumped with my youngest because I always fed her live. Then I got asked to come to a tv show and that was a week later.. no way I could provide enough milk for her so my boyfriend could give her with a bottle. So we bought formula.. and I was happy we did.. it wasnt near enough to get her through the day ...

You are doing an awesome job look at all these months of breastfeeding you already gave them. <3