I found a new job with nice and talented colleagues. My salary got a huge increase. I can't wait to start the new job. However, my notice period is of several months and I completely lost any and all motivation. I want to avoid burning bridges, but I have zero ambition left for my current job and I would rather move on today. I do not look forward going back into that office, and I have no enthusiasm about future plans. I felt uncomfortable before, and even more now that I got another job. How to get through a lengthy notice period, without burning bridges, in these conditions?
How about just doing what you agreed on to do (in your contract, in verbal agreements up to now, in written procedures etc), instead of letting it depend on how you feel? They pay your salary during the notice period, don't they?
Would you want your boss to have the same attitude towards you during your notice period, i.e. Well I don't feel like paying him anymore, because he's leaving anyway?
How about asking what you can do in this period to make a smooth transition for everyone?
If you are concerned about get(ting) through a notice period in good health I can tell you that your own attitude will have a lot to do with that.
There is one alternative I can think of, especially if there is not much to actually do in your notice period: you can propose to end your contract earlier.
You can do either of these:
Do your best, despite being unmotivated. There will be various times in your life where you'll be unmotivated for prolonged periods of time. If you find that you can't push yourself to work properly despite wanting to, you may need to consult a psychologist - they exist for these kinds of issues. These issues are much more common than most people think.
Take unpaid vacation, or end your contract earlier, after discussion with your current employer. The advantage of this solution is that you can travel around the world, probably for months. There's a risk that you will start the new job and still find yourself unmotivated, in which case you may suffer from a burnout and should seek the help of a psychologist .