The famous phrase 'You can't be in two places at once' seems relevant here. But don't lose all hope. Physicists seem to have demonstrated quantum superposition (4). 

While we hold out for scientists to give us the ability to occupy multiple locations simultaneously, it may be helpful to consider a psychological response to this issue.

If you can go along with my analysis, these seem to be the key strategies in coping with Onism:

It's unavoidable

Based on social comparison theory (including the upward bias), opportunity cost, and current physical limitations, we cannot avoid a degree of Onism. This emotion is a normal feature of our existence. 

We can notice, name, and normalise difficult emotional states. Acknowledging the normality of Onism may help you shift your perspective on this state from a problem to 'solve' to a natural feature of life. 

Get mindful

Mindfulness is a helpful approach to cultivating present-moment focus. Mindfulness may also be a way to mentally step back from the frustration of Onism. If you've not already tried this strategy, here is an article on mindfulness and here is a body scan exercise: