Sleep regression or habit?

My daughter is 19 weeks old. She has been sleeping in her crib since 12 weeks, and has never been that great of a sleeper. Her naps were 30 mins on the dot and her night time sleep would vary between 2-4 wakings. She then hit about 15 weeks and I was up 6+ times with her. We decided to sleep train her. I was rocking or nursing her to sleep before this. Now she puts herself to sleep for naps and initial bed time with little to no crying. Problem is she is back to getting up every 1.5-2 hours. If I do intervals and try to get her back to sleep without nursing her it takes a good 45 mins and she only sleeps maybe another hour. So I end up nursing her and within 10 mins she is back to sleep. But we do this all night long. I’m so very tired!! I was under the impression that once she can put herself to sleep her sleep would get better and if she woke in the night she would put herself back to sleep. So, is this the 4 month sleep regression or does she associate nursing to go back to sleep? How do I know when the regression ends, will it be clear? Her naps are great and her first round of sleep is great, but after the first wake up in the night it’s all downhill from there. Help!