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
tl;dr keep pushing yourself, but gently as Paul Sasik says. Also, keep an open mind because this stuff can be very counterintuitive.
Long answer
It sounds like you're practicing the Mahasi Sayadaw tradition, where one notes phenomena as they arise. In this case I can answer from my own experience in the tradition.
A few pointers:

Negative thoughts are just thoughts, so you have the opportunity to note &quot;thinking&quot;.
Ditto for a clouded mind; you have there an opportunity to note &quot;clouded mind&quot;.
Ditto again for guilt; if guilt is predominant then note &quot;guilt&quot;.

The essence of this strategy is not to get enticed by what we perceive. Thoughts aren't intrinsically negative or positive; we mistakenly understand them to be as such. The same goes for feelings like guilt, and the realization that the mind is clouded. When we see them as they are, it's easier to remain in the here and now.
As for the wandering mind, I can say a few more things:

Try to abandon the desire for practice to go well—paradoxically this can hold you back.
Don't curse yourself. Cursing yourself is a form of identifying with your experiences.
Experiment a lot. Vary the quality of your practice—Does one note intensely, or note softly? What can be noted that was not noted before? What are the limits of what can reasonably be covered by noting? Do the limits change from one day to the next? Come up with ideas like these, and test them out during walking and sitting practice. At the same time, keep it simple. You don't want to get too obsessed with conceptual dead ends.

It can take a long time for practice to improve. But I can confidently say that my mind wanders much less than it did one year ago. In my 3 years of practice (one 6-day retreat, about 15 minutes per day on most days) I have found there are long stretches—perhaps days, weeks, even months—where no progress is made, followed by flashes of what seem to be insight, followed by the realization that they are just unusual experiences, and so on.
Lastly, as for maintaining mindfulness throughout the day: I find this one quite difficult. It improves with practice just like the wandering as I explained above. Personally my daily mindfulness is quite weak but at the same time much better than, for example one year ago. Another side of this is that the monastic lifestyle or a meditation course will give you the opportunity to develop these skills more quickly.
Be well.