Excel running total by month

Hello,

I am keeping track of a loan. Each month, there may be one to a few lines added. Some are credits.

I would like to see:

entries for each month 

total of entries for that month

interest on that month's total (simple, not accumulated)

a running total of the loan

a running total of the interest 

bonus would be a grand total of principle plus interest but I could do that manually!

In the attached workbook, I've shown my loan two ways. The first worksheet with the tab "2018" shows me the running total but not summed by month.

The tab "sheet 2" shows the result I would like but I am entering the running total formula manually, which leads to many errors.

I'm using office 2013 Windows 10. I'm not very sophisticated in Excel use. I don't know how to use macros. 

I'd appreciate any help!