When scheduling the meditations into your day, it's important to give yourself enough time.
If you're just setting aside 30 minutes to do a 30-minute meditation, you may be too rushed to even get started. And if you do manage to start, it's easy to get distracted if you're feeling rushed or impatient, or to start thinking about what you have to do next before you're even finished the meditation. Try to leave yourself a buffer of 10-15 minutes to relieve any time pressure.
Meditate even if you don't feel like it.
Even if you feel too tired or distracted to meditate, start anyway and see what happens. You might surprise yourself and be able to concentrate better than you'd expected. And remember, there's no such thing as a bad meditation.
There will be times when you're more focused, your concentration is stronger, and you feel like you're meditating "well." And there will time when you find it difficult to concentrate, you're easily distracted, your mind wanders all over the place, and you feel like you're meditating "poorly."
But remember that mindfulness is about paying attention non-judgmentally, not judging things as being good or bad, desirable or undesirable. All of your experiences are equally valid. A period of meditation while you're tired is a great opportunity to practice staying mindful and maintaining your concentration under less than ideal conditions.
It's relatively easy to stay mindful if you’re relaxed, there’s nothing stressful going on in your life, and you’re outside in nature on a beautiful day. It's much more difficult to do so if you're feeling tired and unmotivated. So those times when you're not feeling your best or most alert are perfect opportunities to practice and improve your mindfulness skills.
And if your mind keeps wandering, and you're constantly distracted, that just means you have more chances to practice noticing when your mind has wandered, acknowledging where it is, letting go of the distraction, and refocusing your attention where you want it to be, for however briefly that may last, even if it's just a few seconds, until you get distracted again and your mind wanders some more.
So try not to be too hard on yourself. Self-criticism and feeling like you've failed are only going to decrease your motivation to do the mindfulness practices. Just be accepting of yourself, and of your experience while meditating, whatever and however it may be.