Hi, welcome to PC. I hope you find support and a community you can benefit from here. I'm new to all of this too. I decided to quit a job I loved while manic about 10 months ago. It was the right thing, but I regret how I went about it.

I think it is completely possible to make good decisions while manic. There is usually truth at the root of what you decide even in that state. I think the difference is in how one goes about making the decision. They are sometimes made with zero calculation of risk and even without consideration of how they affect the lives of those around us. In those cases even 'good' decisions feel bad.

I think it is great that you now see a pattern in your decision making. You now know you make huge life altering decisions very quickly while in a manic state. You can now learn some coping skills to help yourself break the pattern. You can always continue to go with it, but I imagine your good fortune will not always be the case. In the future you can train yourself to slow down. You can stop and ask yourself a few questions before proceeding. Some examples are:
Am I experiencing any symptoms of mania?
How long have I thought about this?
How might this decision benefit me?
How might it go badly?
What are the risks of this decision?
What are the consequences?
How will this decision affect others?
Have I taken an appropriate amount of time to weigh this decision?
Is there any research I should do before proceeding/do I have all the facts necessary to make this decision?
Do I know the right steps to take to implement this decision?
Do I understand how much or how little work will be required of me to do this?

You can then share your answers with a therapist or a trusted loved one. They can help you see where your answers might be off due to the mania. This process might then help you decide when you need to wait a little while so that you can revisit your choice when you're closer to a baseline state. That way you can still capitalize on the beneficial thoughts that come from mania, but you can implement them in a more healthful and strategic way. It should also root out bad decisions easily.

I know this diagnosis can be incredibly difficult, but there are strategies you can incorporate to make it easier. Good luck to you on your Master's program. That's exciting work.