Going strong, don't want to crash

So this is my first official week going all out health and fitness. I haven't cracked once for fast food and totes had the opportunity tonight to but stayed home and made my dinner instead.
Any advice, tips, suggestions for staying on track this way? I've been coming in under my allotted calorie consumption but eating healthy enough and often enough I'm not even starving at night or feeling deprived throughout the day.
My main struggle is that I can tend to go good for awhile and then crash and burn and I usually would have by the 2nd day, sad I know but true, how can I keep this going besides plain ol' "will power"??

Thanks!

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