When I suggest learning about mindfulness and meditation to my clients, some of them are cautious in regard to some app telling them to "let go" or "non judgmentally push out any thoughts." Doing so is tough and can then create more anxiety than you started with. I found a great article on mindful.org that is honest about those who struggle to get anything out of meditation or find that it increases their anxiety. For those struggling with the self criticism that sets in when you are practicing meditation/mindfulness, here is a nice step by step manual in getting yourself to feel more comfortable during your practice.
1. Sit upright in as comfortable a position as possible. Eyes can be open or closed—whichever is more comfortable.
2. Silently begin with a recognition of the reality of anxiety. Make the words your own, but quietly say something like: “I’ve suffered a great deal. This pain is real and intense.” Place your attention on the words, and repeat them quietly a few times.
3. Place your attention on a single breath—feel the air coming in, and feel it leave the body.
4. Silently repeat the phrase above and consider adding the following: “In this pain, I’m caring for myself.”
5. Now try placing your attention on two full cycles of the breath, feeling the sensations of the air coming and going.
6. Add the following self-compassion anchor: “This is hard, and right now I’m giving myself permission to understand that.”
7. Expand the practice out to mindfulness of three to five cycles of breath.
8. Say to yourself: “Though the pain continues, may my practice and care for myself continue as well.”
9. Continue in this way, allowing self-compassion to anchor your practice of mindfulness. Let it be a scaffold on which to stand in self-acceptance, and let it help you disarm the inner voice of criticism and failure.