There are a lot of different practices out there, often given the general label of 'meditation' though they are very different in their intentions and results.
I teach insight or mindfulness meditation, otherwise known as Vipassana meditation, and the fundamental principle is 'Be present.'
What you are practising is the ability to come into the present moment. In the process you will inevitably discover a number of thoughts and attitudes within you: the restless mind.
You don't have to do anything about these thoughts, just notice them and keep practising being in the present moment. Solutions, beating yourself up, fighting your discomfort, anything that sounds like hard work in the least - none of this is necessary. The process resolves issues on its own. You may find your own natural solutions and you will certainly gain some important insights, but they will not be imposed on you.
Any tension in the body has a corresponding tension in the mind. These tensions can become habitual and we may even think of them as being 'me'. However, they are merely thoughts, merely processes, usually based on a sense of fear and separation, and observing them is all that's required to dissolve them.
Good posture is helpful because it allows the body to start letting go of these tensions. Good posture, however, does not mean 'making yourself more tense in an effort to sit up straight.' It's about finding a sense of comfort and ease in the body and it is a natural way to move and sit.
Playing around and allowing the body to find this sense of comfort and ease is useful. If your sitting posture hurts, change it and find something that feels good.
Forgiveness and loving kindness are practised in mindfulness because a sense of peace and relaxation is not possible when we are tied up in ideas of revenge, self-protection, suspicion or separation. We are built on relationship, our happiness is dependent on it, and we all have not only suffered but have caused suffering to others. Forgiveness is the way to heal these injuries and allows us to move on to better ways of living.
However, forgiveness is NOT the same as 'allowing a bad situation to continue.' This is a common confusion, but is also unloving to you. It is possible to be relaxed and calm but still have the strength to make your life better for yourself.
Full, clear breathing is important so that we can allow our emotions to move through us. There is nothing wrong in any emotion we have, they are natural and helpful. If we habitually suppress or deny our emotions (for instance we may have been taught that anger is 'bad') they may become dammed up and have to start shouting very loudly to get our attention. In this situation we will also cope less well in life because our natural instincts are suppressed.
Therefore emotional releases are to be expected and celebrated. They represent a softening in the body as we let go of our stored emotional blocks and issues.
Breathing deeply with the belly dissolves a lot of anxieties. We tend to breathe with the chest and hold our stomachs in which creates unnecessary tension in our mind. Retraining the breathing so we can breathe comfortably into the belly can help reduce a lot of mental noise.
Being able to relax at will makes life happier and problems become smaller. There's more room for fun, enjoyment and ease when the dominating sense of fear and separation begins to dissolve. Everything really is OK - you may just have forgotten that temporarily.
I do not underestimate the difficulty of being human or the courage required to sit with oneself. As a meditator -well, as a human being! - you are demonstrating great courage and you are doing it alone. These subconscious forces can be tremendously uncomfortable on occasion, and sometimes it can feel impossible to find our way through them. However, with patience, persistence and practice, every cloud clears eventually.
Mindfulness meditation is a great way to start discovering these truths for yourself. The course is built around these fundamental principles.