What's done is done. There's nothing to gain from beating yourself up over a mistake made. Negative feelings could turn your slip-up into a much bigger thing; you may feel like ditching the plan entirely.
Think about why your slip-up happened. If you know, you can stop it happening again.
Maybe you ate unhealthy foods because you were bored or upset, or you missed your evening walk because of something on the telly. Find the reason why you went off-track.
Think about what you'll need to do differently next time to avoid making the same mistake.
Next time you're bored, could you phone a friend, read a book or take a walk instead of snacking? If you don't feel motivated to do any exercise in the evening, perhaps you need to buddy-up and find a partner who will do it with you.
There is no need to dwell on the slip-up now that you have a plan in place to deal with it next time you are tempted. Move on, and expect that things will work better for you this time.
Are you having regular slip-ups?
Having support is also really important for success. Try to get your close family and friends on board to support you.