Can't meditate?

Deliberately directing or focusing the mind onto the breath is not the actual practise of Buddhist meditation, despite this wrong 'yogic' technique from the Visuddhimagga being taught so extensively. Instead, the genuine practise of Buddhist meditation is maintaining vigilance (mindfulness) over the mind to ensure the mind (mindfully) remains in a state free from craving, i.e., free from covetousness & distress, as defined in the suttas, as follows:

The monk remains...mindful — putting aside greed & distress with reference to the world. Anapanasati Sutta

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There is the case where a monk develops mindfulness as a factor for awakening dependent on seclusion, dependent on dispassion, dependent on cessation, resulting in relinquishment (vossagga). Anapanasati Sutta

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And what is the faculty of concentration? There is the case where a monk, a disciple of the noble ones, making it his object to let go (vossagga), attains concentration, attains singleness of mind. SN 48.10

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One is mindful to abandon wrong view & to enter & remain in right view: This is one's right mindfulness. MN 117

When mindfulness continuously remembers to keep the mind free from craving, due to having a clear mind that is not pre-occupied with any sense object via eyes, ears, nose, tongue or mind, the in & out breathing will automatically arise as the object of meditation or 'sign' (nimitta) because the breathing by default becomes the most gross or dominant sense object of the mind.

In your personal situation there are a number of matters to discuss:

1. Based on what I posted already, when a meditator deliberately focuses on the breathing (in the same way that is taught to sportspeople to control their breathing), that deliberate focusing is an act of intentional thought. That intentional thought will always remain in the mind. While that intentional thought is more subtle that an ordinary thought, it remains relatively coarse. This intentional focus on the breathing will cause the breath to calm. However, once the breath calms to a certain degree, because the coarseness of that intentional thought remains the same, the mind suddenly becomes too coarse to discern the breathing that has now become more subtle. Thus, the meditator can no longer discern the breathing clearly. An analogy is jogging vs meditating. The breathing is easy to discern when jogging because the breathing is coarse. But, generally, when meditating, the breathing is difficult to discern because the breathing is subtle when sitting. If the breathing is calmed by deliberate intentional focus on it, it will become even more subtle & difficult to discern. THEREFORE, the only way to continue to discern the breathing clearly is to make the mind more subtle. ULTIMATELY, the only way to progress fully on the path is to abandon the deliberate intention to watch the breathing, thus making the mind as subtle as possible.

2. When the bundle of negativity exploded in your meditation, this was the result of your previous concentration upon the breathing & the resultant calming of your breathing, body & mind. The explosion of negativity is a typical purification that will be experienced many times on the meditative path. As a result of this explosion of negativity, there arose some 'momentary rapture', which was the pleasant feeling. IMPORTANTLY, related to what I described in detail above, this explosion of negativity made your mind more pure and therefore more subtle. Further, the mental energy moved up into your brain as a temporary part of this purification process. Here, the energy must be allowed to balance & dissipate. You must give the energy the space & freedom to dissolve. This is done by simply giving up craving; by giving up your positive & negative judgments about the experience. If you can maintain the mind in this state of non-craving, non-judging, non-coveting, non-distress, it will connect again with the breathing on its own accord (and will periodically disconnect on its own accord when there is other purification work for it to do).

3. In summary, it sounds like your meditation retreat experience was very successful because your mind could concentrate and a strong purification (catharsis) occurred. This shows your mentality is pliant & can let go, which is a favourable quality for meditation.

4. I personally experienced these issues in my 1st five weeks of meditation (which was full-time retreat meditation). While my mind never had any hindrances or distracting thoughts and remained relatively calm due to my first two weeks of meditation, the ability to discern the breathing clearly kept disappearing. It was only after I changed my technique to giving up all positive & negative motivations & judgments, did the breathing become & continuously remain exceptionally clear to the mind. The Buddha's entire path can be summarised as the abandoning of craving.