Give up or keep trying?

So I have had 2 ultrasounds since 16 weeks (I am now 18 1/2) to find out the sex of my third and last baby but baby wouldn't show us either time. So I'm trying to decide if I should just stop trying and let bean surprise us... I just worry that if I do that when I give birth if the midwife says "it's a girl!" (I have two girls and really really want a boy) that I will be disappointed and I don't want to be sad when I first meet this LO. If I find out sooner I can have time to get excited and get over the gender disappointment... Thoughts? I'm kind of lost on what to do:-(

I thought that I really really wanted a girl with my third (I had two boys at the time). They were able to tell on the ultrasound and when they said it was a boy I was surprisingly very happy. Don't worry, you will be in love right away if it is a boy or girl. I never felt let down or sad. I just think seeing that little person took all the longing for a certain sex away.