Oh wow, definitely do not feel guilty. There never needs to be guilt, for any reason. If you had simply chosen to ff or to mix feed then you wouldn't need to feel any guilt.
However you have additionally had a huge impact on your physical state, and no doubt emotional state - every birth will do that! But you have that added impact from undergoing a traumatic event and major surgery. Go easy on yourself, you have this beautiful baby, you've done something amazing, all you need to do is rest up and take care of yourself and the baby, ideally with help from others too if you have it. Taking care of the baby just means warm, dry, clean and fed, and most of all loved, whichever way you reach those states is not important.
As for your question at the end, a lot of mothers mix feed and face those difficulties establishing bfing. Some continue to mix feed, some fully establish bfing, some move exclusively to ffing. After I had DC1 I tried bfing, expressing, then tried topping up with formula... long story short my milk never came in, DC1 was losing weight and becoming dehydrate, I decided to ff exclusively. A good friend had an EMCS and mix fed almost from birth, it took her within a month of the birth to establish a supply of breastmilk and she decided to continue mix feeding which she did without issue.
Have you had visits from MWs or the HV? They can help with anything immediate but they will also know about other support available in your local area. There are also breastfeeding groups that will visit at home if you wanted to talk it through and get further advice and support for bfing. Hopefully you have others around you to support you too. When you feel up to it there will be groups at local children's centres, support cafes etc where you will be able to meet other mothers and hear their experiences and that might be helpful too, just to have some RL support from people who can relate. And of course, MN will always be here 
