Oh, I know exactly how you are feeling, hugs to you!!
It is a really weird feeling the first time you give formula, I cried!!
Like you and PP it just wasnt working for us due to extreme stress as my husband had an accident at work when DD was 3 weeks old. Slowly I began to lose my supply and DD was so upset all the time. I tried everything but then gave her a formula bottle and she was a different baby!! That was the decision made. Do try to keep at least one BF, that is my only regret.
It is a very sad and guilty feeling and just know that when you do stop breastfeeding or cut down your hormones might change a bit and you may feel even more upset or depressed. But that will go away. I found I could enjoy my DD sooooo much more not having to worry about how much she was getting. She is now an extremely happy, healthy and beautiful 7.5 month old.
Breastfeeding is such a hard job, I didnt realise how hard and whatever you can give your LO is great
Hugs x