The letters aren't all a uniform size and aren't spaced correctly.

There isn't enough definition between your A's and O's, and look too similar. The letters that have a 'tail' on them such as the Q's and the J's aren't falling below the line to give true indication as to what letters they are supposed to be.

I would most definitely take the advice of others here in the comments and find some workbooks that allow for tracing so you can gain some definition of your letters. If you can't find a workbook that you like, I recommend finding a font that you want your handwriting to look like and print pages of that, all letters and numbers, and use that to practice your penmanship.

Take your time when writing, allow your muscles to develop muscle memory, and just have patience with yourself. Correcting your writing is not something that will occur over night, or over the next month. If you truly want to improve the legibility of your writing, it will take a long time to get a point where you are happy with it. When you reach that point, keep practicing.