Am I fat and other thoughts

I can't believe what this weight loss/weight gain journey does to our heads.
I've lost 13 lbs since Christmas, so I feel quite skinny.
But I am now only at my starting weight for Weight Watchers 3 years ago when I felt enormous and swore I would never get this big again.
I "only" weigh 135lbs, but at 5'1" I'm overweight.
People tell me all the time that I'm not fat, but I have a hard time trusting them. Is it because they're much heavier than me? (I know that's not kind, but those are the people who usually say that).
I look at people who've gone overboard on their weight loss and they look like they have a terminal disease and I think, can't they see they need to gain weight back.
On and on it goes.
When I was a teenager living in England, I hardly paid any attention to my weight, walked a lot, ate what I wanted and stayed skinny.
Since moving to the States, I've become obsessed with my weight, gained weight and always feel like I'm on a diet or should be. Of course, I'm no longer a teenager, far from it. And the weight I was then (around 100lbs) wouldn't even look good on me at 50 years old, but part of me still feels fat if I'm over 110lbs.
So, what is a good weight for me?
And what weight is sustainable without driving myself crazy?
I'm trying for my weightwatcher's goal of 125 which means another 10lbs to go.
Wish me luck..