My name is Hank I am 26 and I have a facial PWS.
When I was your age I could diffently be in your shoes with out so much self esteem issues. My mother probably had more of a problem with my birthmark rather then I did.
Feeling alone in relationships I totally agree with. I did not have my first relationship until I was in college. Just remember people see you for how you see yourself. So if you have a problem with your birthmark everybody else will. Do not get me wrong you know as well as I do we both will get asked about our birthmarks everyday for the rest of our lives, but if you can turn it into a positive some how it really does help. For example children I turn it into education. You will always get the rude people though. I work as a vender in Wal-Mart in sales so you can picture how many people I come into contact with.
I guess what I am getting to is you are the one that has to except yourself for who you are before others will except you. That will build your self esteem. If someone asks you a rude question just tell yourself that they are just making a fool out of themselves for not be educated enough to know what a birthmark is.
As for the relationships. You have to be confident in yourself and step out of your boundaries and ask the right person out at the right time, and try looking outside the box and lower your expectations if you think you are being picky. I met my fiancee two years ago at a vision impairment school where I was learning how to be blind. She has been legally blind her who life.
I hope this has help a little. If you have any other questions for me you can either message or email me.