This has happened to me a lot recently and I'm going to tell you guys this from experience, don't wait to approach a girl once you see her. For instance, I saw Amanda afterschool after her swimming practice, which is rare and she was sitting there alone and even said "Hey" to me once. But I just kept pacing the hallways saying "I'll go up to Amanda when this or that happens, I'll go up to Amanda when it's this time or that time...". And you know what happened, I came back, Amanda's guyfriend was there and she left. So basically, I had the perfect oppurtunity to approach the girl, but I didn't take it and now I just freaking regret it. So basically, once you see a girl you want to approach, just go up there. Don't make excuses or wait when you see a girl you want to approach, go up there the second you see her. If she's with a guyfriend, forget about that approach her. And another way to get better at just going up there is, just say anything, don't wait and think about what to say, because by the time you've found your answer, she'll be gone.
