Falling in love with my best friend

Hi guys ive been single for a few months now and my best friend of 8years has been there for me through it all hes helped me out around the house with some diy took me shopping helped out with my three kids took me out for breakfast a couple of times etc i love spending time with him and cant remember the last time I felt this happy, we had a moment of madness and slept together (protected) it was wonderful and today we spent some time cuddled up together kissing. The only problem is hes engaged! I feel awful about it and so does he but we can't help the way we feel we don't mean to hurt her and thats certainly not what we want but we have no idea what to do about it?? Please im asking for help and advice here not opinions thanks