relationship advice?

my babys father & i took a break for 3 days due to arguing a lot from me being overwhelmed about being a mother. He went to stay at his sisters & we didnt talk for a day. the next day he texts & asks about the baby so i send him a picture. he then proceeds to tell me how much he loves me & misses me & wants his family. So i go onto his instagram account & i learn what i never wanted to.... he was talking to another girl. Not only was it a girl he had a 3some with 2 years ago but he didnt even know her name just her instagram name & he said that he wanted to be with her. A girl whos name he doesnt even know... I told him i was leaving for good if he didnt delete the account & change his number & he did. I took him back after he did that but i still cant get the picture of it out of my head.. as for the other girl he didnt contact her. We have only been together for a year so he wasnt with me when he did this. He didnt go see her or anything & he got on his knees & cried & apologized but i still dont know.. i feel wrong for taking him back but i love him so much i just had mine and his first child in July.. He was begging for me back & said he wasnt in his right state of mind because i "threw him out" when i said we needed time apart. what do i do i want to forgive him but all i do is think about it.. ADVICE PLS.