Rising heart rate and severe withdrawel symptoms after just a few doses of mirtazapine?

My GP prescribed mirtazapine 15mg for insomnia that I'd been dealing with for a month.

My dosing timeline:

• Started January 6th
• Jan 6-7: 7.5mg
• Jan 8: 3.75mg (split tablet)
• Jan 9-13: Stopped (severe rebound insomnia, extreme exhaustion)
• Jan 14: Took 15mg once due to inability to function
• Jan 15-22: Off medication
• Jan 23: Took 15mg (heart rate hit 100 from sleep deprivation, felt desperate)

Current symptoms:

• Resting heart rate jumped from average 69 bpm to consistently 94-100 bpm (even when relaxed)
• Shortness of breath
• Dry mouth/cotton mouth
• Cold chills
• What feels like blood pressure fluctuations

Last night while calmly watching TV, my watch alerted me that my HR was 99.

I called the after-hours GP service and they said these are side effects and to wait until tomorrow to see my regular GP. I've only taken 4 tablets total over 18 days, but I'm experiencing what feels like severe withdrawal symptoms when I stop. The rebound insomnia is brutal and I'm caught in a cycle where I get so sleep-deprived that I feel forced to take it again.

I'm seeing my GP tomorrow to discuss discontinuing this medication properly, but I'm genuinely concerned about these physical symptoms - especially the elevated heart rate and breathing issues.

Has anyone else experienced these symptoms on mirtazapine? Is this normal or should I be more worried? Any advice on managing discontinuation after short-term use?