Yeah, D. - you're so very young - things can still happen for ya!!
My first bf happened at 19 too, and when I really wasn't expecting it!!
You might wanna change your nickname for a start? And maybe start looking at yourself with different eyes?
I used to be very much bothered by happy couples too, someone online mentioned a trick to try to imagine yourself with someone great at that time too? Might wanna try it, if it works? (Maybe with pictures first?) Or you could see if great music in your ears helps, or such? (If it's on a walk/jog?) Or talking to someone who's with you?
You know those couples may split up later, or maybe they just quarrelled too.. but are just like that in public.. Some of my friends are divorced now.. (and they used to be all 'lovey dovey' too..) But it's still difficult when you're 19, and it's Spring/Summer outside, I know..
Someone I know who is really great was single for a loong time, then met her man at 24 and they are still happily married!!
Also check if you might have gluten sensitivity, or some other food allergy/sensitivity.. or if sugar/soda pop intake could be a problem.. Some people can have these as cause of depression/anxiety or crying too..