I've been encouraging my daughter to accept the way she is no matter what anyone says for a long time. But it's not always about loving the physical you or knowing what you like and being confident and how many friends you have. Those are important and we may think kids are so good at being themselves without a care we are forgetting that some kids are sensitive to others and are not selfish enough to not get persuaded and manipulated into being taken advantage of.
Once I saw that my daughter was being weighted down by all this emotional turmoil, I thought "she's not ready for relationships further than casual friends." She's only 13 and needs more guidance. I've come to realize that parents do need to monitor and guide kids in relationships and talk with them about what good relationships are and what to watch out for in case it turns ugly. Parents do need to step in when something is not right. Or may be end it entirely if it's completely unhealthy.

Here's somethings to help guide your kid into loving themselves and learning how to take care of themselves so they will be able to have better relationships with friends, lovers and this will help prepare them later on, when they start their own families:

1. It's okay to be selfish. - When I was new mother, I put some much energy into just focusing on my baby. I thought that's what good mothers do. But then my Home Visitor told me that it's just as important to take care of myself.

2. Explore yourself. - Not just physically but emotionally, mentally, spiritually. Learning all you can about what makes you you and finding acceptance will bring confidence and self-esteem knowing your tastes and tolerance.

3. Relax. - Taking on less things to focus on keeps you grounded and more able to succeed. Pacing yourself will teach you to be patient and have better understanding of your limitations.

4. Talk to trusted adult. - It could be any trusted and respectful adult in your life, even if it means making calls to the Kids Help Phone. And speak up the moment you start feeling "off" or if something doesn't seem right. But also make an effort to talk to someone when everything is okay.

Loving themselves will become second nature later on. This may or may not lead to an ego, but a genuine ego is better than a fake forced one.

Of course, hormones are going to get in the way. I'm not saying that they should not have crushes. That's just inevitable. I'm talking moreso about serious relationships or dating when they are not emotionally prepared. Teens need to learn to listen to their own needs and not follow the pressures and standards of what others think they should be ready for.
On that note, 13 year olds (teens in general) are not little adults. They should retain their innocence for as long as possible and be taking on responsibilities little by little and plan for the future with their education and prospects of careers.

(I hope this makes sense. I tried not to make this too long.)