a) I love you and must represent you immediately or
b) Get away from me you mad person or I'll call the police.
Now imagine you're an agent receiving the equivalent in covering letter form. Straight to the top of the Must Be Read pile? Or the Immediate Rejection pile?
So how do you say I'm wonderful without saying it? The simple answer is you get someone or something else to say it BUT it's got to be the right someone or something. So your Mum is not the right someone and nor are your children nor is anyone who has any personal connection with you, because of course they think you're wonderful, but their opinion doesn't mean anything in this particular context.
I'm not convinced an author or a creative writing tutor is much good either: 'Joe Bloggs suggested I write to you' doesn't mean much when it comes down to it. If Joe Bloggs really rated your work, they'd snatch it out of your hot sticky little mitts and personally hand it over to their agent/editor.
The person you want to endorsement from doesn't know you. They only know your writing and, ideally, paid you money for it. For example:
They gave you a prize in a short story competition.
They published your article.
They broadcast your short story.
They published your non-fiction book.
They bought 1000s of copies of your self published book.
The more credits you can build up, the more endorsements you're getting. When I was at this stage I deliberately entered every short story competition I could find to build up some endorsements. When I wrote my covering letter I was able to say I'd won or been short listed for seventeen short story competitions. (Which, thinking about it, also shows persistence and a degree of obsession that is very useful for a writer.)
I'm not saying that you HAVE to have credits, but credits mean someone else picked your writing out of a crowd. It will give an agent confidence in you and your writing.