Trouble Maintaining Practice

Okay, so I searched for an answer to this question and couldn't find one, but it must be common, so I apologize in advance if this is a duplicate. I try to practice daily mindfulness by saying in my head, "walking" as I am walking, "reading" as I am reading, etc., but often I lose my focus, and three hours later I find myself thinking, "oh yeah, right, I was supposed to be practicing mindfulness.." This happens especially when something I perceive as really bad occurs- negative thoughts cloud my mind and take me over. And then I feel SO GUILTY! How do I overcome feelings of guilt about my practice? And how do I sustain mindfulness throughout the day? It seems almost impossible, and every time I realize I have failed at it, I curse myself and feel like a hopeless buddhist. Thx for reading! Ian