I just need to know if anyone has been through this or something similar and what to do.

At 3 months old my daughters father and I broke up. Now she’s 3 and he wants her. I don’t know what to do. I want to be the bigger person and I want to allow him to see her but i’ve been alone with her for 3 years when no one else was there and i’m 18 now and i’ve always been the one that gave her 100% of my time, energy and love, and now she’s going to be with someone else and I don’t know what to do. She was with him for a few hours last week and she already loves him so much and i’m scared that she will let him in and then he will just abandon her again. I DONT KNOW i’m so confused. She’s my baby and i’m the one that shows her unconditional love and i’ve been there for her and I don’t know. I’m jealous that she can love someone more than me. I’m worried that he will just break her heart again. And i’m scared of losing her. When I picked her up from her dads house she ran up to me and hugged me and was like I was a stranger to him. I want to scream and cry because he wants full custody over her but she my daughter. I was with her when no one else was and I understand that he’s her dad but i’m her mom and i’ve always put her first and he hasn’t.