Don't beat yourself up about it, this stuff happens. Here are some good beginner articles and resources to help you not repeat your mistake. If you are going to keep your next tree in a similar situation, my advice would be to water it in your sink and let it drain completely - that coffee cup will just hold the water and it will wick back up into your pot and act as if you have no drainage.
Start here and move on to the articles link at the end:
https://www.evergreengardenworks.com/bonsaip.htm
Another good springboard for a beginner. The paid beginners course videos they offer are good too:
https://www.bonsaiempire.com/basics
You could also check with your college library and see if they have the following books that could be had via inter library loan:
"Bonsai" by Peter Warren
"Bonsai Basics" by Harry Harrington
"Bonsai Workshop" by Herb Gustafson
"Bonsai Beginners Bible" by Peter Chan (this one may be dated)
You can also search YouTube for "Bonsai Beginner Series" by Bonsai Mirai.
This should keep you busy for awhile.