Screaming at sleep times

Hi ladies!! My little sweet pea has such a rough time at sleep times. I know separation anxiety is totally normal but I feel so bad. We started sleep training in August because she showed signs that it was time. Now every time I’ve put her to bed since then she screams when I leave the room. Her pedi said that it’s completely normal and she’s perfectly healthy. But it’s been going on for four months. Everything Ive read said that babies get into the routine after a week or two. We have a routine we follow and stick to it. But it doesn’t matter how long or short of time I spend doing it. She screams every time then after 5-10 or even 20 she falls asleep. Is this normal for littles? She’s going to be 15 months soon if that helps.