First thing's first. You tried. You submitted that book, you put in you resume, you walked up to the cute person in your office and you tried. That's the first and most important thing because it shows that you have guts. You knew that there was a chance it wouldn't work out but you still did it. Good for you. That's awesome. You're awesome.
Moving on. You also have quite a bit of patience. Most of the time a rejection doesn't come instantly. One rejection I received last month took five months to get to me. It was frustrating and I was annoyed. But I tried it and it didn't work out. That's fine. Now we have to keep our chins up and move on because rejections often take time. Also, they usually don't come with reasons why you were rejected. Very few of the publishers I have ever worked with give reasons why a book didn't work for them so when you get the publisher that does have the time to do that for you, go through their suggestions and see what you like and what you don't. They're opinions but they're often pretty spot on.
But at the same time don't throw out everything for the sake of pleasing one house. There are some things that you may not want to change and that's fine. Unless its one of the big things most publishers won't touch then you need to make your own judgement about it and go from there.
If the house gives you the option to resubmit then consider what they said and decide if you want to do it. Or move on to the next house. The joy of publishing is that there are so many publishers available and each offers their own unique benefits. More on that in another post though.
Rejection letters remind us that we tried, teach us patience and keep us humble. I play with imaginary people in my head all day. Its nice to be reminded that I'm not an all powerful being sometimes. Not all the time of course but sometimes. I'm still learning. We all are. And what works for one house may not work for another. So before you slash through your story take an objective look. Have someone whose opinion you trust look at it and have them tell you any issues they see. This is especially helpful if they don't know the reasons you got rejected- if any were given.
Hearing No once in a while is part of publishing. You're not going to be perfect forever and sometimes there are stumbling blocks along the way. The point is to keep trying. If you want to be published then go for it. The world needs your voice. But know that there is always the chance of a rejection letter there and have the resiliency to stand back up, brush off your pants and try again when you get them. It took guts to submit. It took dedication to finish your book. And one rejection letter or even five will not kill your dream of being an author unless you let it. Own up to your choices and embrace them because ultimately this writing thing will only happen if you put the work into it and really want it.