1. Don't expect to be an instant success. Rejection is hard to bear but your mistakes will teach you something. Every trade has to be learned and learning is often painful. Keep trying, don't give up and in time you'll get there.
2. Pick your market -- the publisher who produces the kind of books you most like to read. Most publishers have a tip sheet, dealing with such things as length, target market, popular themes and the best way to submit manuscripts. Read the latest books the publisher has brought out -- not to copy but to get the 'feel' of what is wanted. Only submit then.
3. Make sure your manuscript (of which you have copies) is perfectly, professionally, presented. Include a short biography.
4. Set yourself regular times in the week when you must write.
5. You're writing a love story. This is always easier if you are, have been or about to be in love. (But keep writing.)
6. In a romance, plot isn't all. Emotion is. The conflict between heroine and hero should be internal, not external -- though it may manifest itself externally. The reader is more interested in what the characters feel than what they do.
7. The first sentence, the first paragraph, the first page sells your book. Grab your reader's attention at once -- but make sure it's relevant to the rest of the book.
8. Be focused on the story. Move the plot onwards. Don't have long wordy bits of description that might be beautiful but are usually irrelevant and possibly boring.
9. End each chapter with a cliffhanger. Some incident that makes the reader want to carry on reading.
10. There may be only a limited number of possible plots. But your voice is unique. Learn from other people but don't try to write like them.
Good luck! You can do it.