I don't know how to feel about this..

So I'm 6weeks postpartum and I found out the my boyfriend is texting with other ladies. One that I know of for sure bcus I went in his phone (I know I know shame on me) but it seem as if he cant let this "High school" love go. They have been in touch for years... (Even before our relationship started). But I seen that recently, she has been sending him nude photos and he asks for them. She lives on the other side of the Country, has a young child herself. I'm just a jealous person already and after having my baby girl my self esteem is shot!! I dont feel threatened I jus feel like its unfair, disrespectful, and plain out bitch of him to be having this inappropriate relationship with this girl. We're not married, so is this what the "girlfriend" has to deal with... ??