Not understanding the practice of anapanasati can be a problem. If confused, just follow the basic instructions or seek advice in a personal interview.
Inconsiderate noise, and other distractions caused by people can seem to be to blame. Merely continue to be mindful of what you are doing. You cannot influence what anyone else is doing in any way except by allowing them to observe your right effort.
Distractions caused by mosquitos and other creepy-crawlies should be treated in the same way. Just continue to be mindful of what you are doing.
Loss of concentration, through tiredness or intrusion of thoughts can be frustrating. This is part of the meditation experience, and these things will happen. Try to continually return the focus of concentration to the breath and accept the distraction. Becoming angry at yourself for not practicing tightly enough will not help you.
False expectations can easily be fixated upon through the full explanation of anapanasati, and through advice from others, including websites such as this one. Try to keep to the basic instructions and accept what happens as it happens. Meditation cannot be forced.
Some days will be good days, and other days will just be difficult ones. Though there is always a root cause of this but it's not really important. It's just a distraction and will only affect your meditation practice if you chose to let it.
In short, everything is just a distaction. Things come into awareness, remain for a length of time, whether short or long, and then they stop. You react in a certain way and can either chose to hold onto that reaction and let it affect you, or let it naturally fade away and continue with your practice.
