That's why emails like this turn me off and almost always result in a rejection:
Thank you so much for letting me submit to you I know that you are very busy and probably don't really have the time to look at my manuscript. I understand if you decide to pass.And yep, I actually receive emails like this all the time.
I get how stressful submitting is. I get that you worry that it's not good enough and that the agent might be too big for you or have a client list that's too big for you. But you also need to remind yourself that you are a contender too. If you want other people to treat your book like it has real potential and is worthy of publication than you need to treat it that way as well.
Stand by what you've written and if you can't, maybe you need to rewrite it.