How to deal with increasing frustration and discomfort while on notice period?

Global context: I am a young-graduated computer-scientist working for an engineering company. My workplace is at the customer's since I was hired 8 months ago. This job has been a filler job after my Master's degree since I will enrol in a PhD programme in 3 months, and therefore leave the private industry. I resigned recently, and my remaining notice period is a little less than 3 months. My hierarchy was very cordial when I resigned and everything went smoothly. Issue: It has been a few weeks I am really sick of my job because it is boring, and I face major setbacks each couple of days, that are mainly due to external causes. Those bad news are piling up with recent disappointments about the uselessness of our team's last semester work. I have a colleague that is in charge of miscellaneous project monitoring duties. His behaviour is openly bossy, but I ignore this behaviour because he has no hierarchical power on me, and he usually agrees with the solutions I propose. There has been a heatwave recently, so I wore open shoes (not flip-flops) and I arrived at the office with a bermuda — in order to avoid looking like a rotting swamp — that I changed on arrival. That bossy guy reported to my manager (without advising me that my arrival clothing could be perceived as inappropriate — in a 30°C office building), who ordered me to stop wearing those clothes for corporate reasons (that are not written in my contract). My manager also suggested — in managerial newspeak — that I should do unpaid overtime. I, therefore, had an argument with both my colleague and my manager, and those events add to the bad atmosphere related to my working tasks. Hopefully, my relationships with all my other colleagues are great. Question: Those last days, I started to be physically uncomfortable just by seeing my office building on arrival. This feeling pushes me to ask the following: How can I deal with that permanent and increasing feeling of frustration and discomfort I undergo while I am on notice period?

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