For ānāpanassati-meditation: don't care the others, even temperature at nose tip. Just focus only breath at nose tip. Even the others come to your senses or your mind, ignore them all. Just focus on only this:

1. "When he breathing in long, he contemplates 'I breathing in long'; Or when he breathing out long, he contemplates 'I breathing out long'.
2. When he breathing in short, he contemplates 'I breathing in short'; Or when he breathing out short, he contemplates 'I breathing out short'.
3. He trains himself 'I am breathing in and sensitively contemplating the entire breath in'. Or he trains himself 'I am breathing out and sensitively contemplating the entire breath out'.
4. He trains himself 'I am breathing gentle breath in'. Or he trains himself 'I am breathing gentle breath in'.

http://tipitaka.wikia.com/wiki/Satipatthana_Sutta#A._Body

Nimitta always appears to every practitioner, however don't care of nimitta. Just keep focus on breath.

For ānāpanassati-meditation: don't care the others, even temperature at nose tip. Just focus only breath at nose tip. Even the others come to your senses or your mind, ignore them all. Just focus on only this:

1. "When he breathing in long, he contemplates 'I breathing in long'; Or when he breathing out long, he contemplates 'I breathing out long'.
2. When he breathing in short, he contemplates 'I breathing in short'; Or when he breathing out short, he contemplates 'I breathing out short'.
3. He trains himself 'I am breathing in and sensitively contemplating the entire breath in'. Or he trains himself 'I am breathing out and sensitively contemplating the entire breath out'.
4. He trains himself 'I am breathing gentle breath in'. Or he trains himself 'I am breathing gentle breath in'.

http://tipitaka.wikia.com/wiki/Satipatthana_Sutta#A._Body

Nimitta always appears to every practitioner, however don't care of nimitta. Just keep focus on breath.

Target of concentration meditation is diverting the attention from unwholesome consciousness's object to wholesome consciousness's object, by ignore the other objects and focus just one.