I can't dare to guess how he trembled when he got the first letter. I fail to remember the exact moment when I got the first letter, but there was the impressive letter in my lifr. It was written by the x-boyfriend. He wrote the phase "I love u" at least 1000 times. Unfortunately he fell in love with my best friend. -> lifr (typo) to "life". x-boyfriend to "ex-boyfriend" (ex means former...ex-military, ex-girlfriend, ex-husband, ex-wife)
"but there was the impressive letter in my life" I don't quite understand your meaning here. Perhaps "but it was the most impressive letter in my life" or "but it was the most memorable letter in my life"
"the ex-boyfriend" -> "my ex-boyfriend" (sometimes you will hear people say "the ex-boyfriend/girlfriend/wife/husband" but that is casual/informal and often with someone who is familiar with that person. People will use it with a not ex-partner as well. "The wife is calling, I gotta answer this." "Ugh, the ex is at it again." It's usually to a friend and it really means "my")
Phase (typo) -> "phrase"
Harry got the worn-out uniform which had belonged to Dudely. He must have felt good as he wouldn't be with his cousin at school. He got the first letter for his entire life. Uncle Vernon deprived him of the precious letter just before Harry opened it.
The second two sentences are a bit confusing. I'll assume Dudely is Harry's cousin, so perhaps you meant...
"He must have felt comforted by it, knowing he wouldn't be with his cousin at school. He got the first letter in his entire life." (first and last sentences are fine)