All writers make mistakes as they put ideas on paper. Because the mind works faster than the pencil (or the computer), a moment’s break in concentration — when someone talks or your phone rings — can lead to errors. Making such mistakes isn’t bad — you simply need to take the time to find and correct them.
Proofreading does take patience but is a skill you can develop. For instance, when you simply glance at the spelling of environment, you may miss the second n. When you read normally, you usually see only the shells of words — the first and last letters. You fix your eyes on the print only three or four times per line or less. When you proofread, try to look at the letters in each word and the punctuation marks between words. Slow down and concentrate.