I need advice

So my fiancé and myself just found out that our son is my ex’s( we knew there was a possibility before he was born) but anyways ever since my Ed came back into my life seeing my son I’ve had mixed feeling for both of them. My fiancé and me currently have a 3 year old girl and I’m 15 week pregnant right now. We bought a house in January and got engaged in January also. I keep telling myself it’s way to much to just give up but I don’t wanna do my fiancé wrong all at the same time having mixed feelings I don’t feel 💯 with him like I used to before my ex came along.

A little back story. My fiancé now we have had a past we started dating in 2012 last strong a year and just off and on till 2015 when I had our daughter. Then I got with my now ex and we was strong for 2 years he helped raise my daughter and then we needed things on crazy and simple things in 2017. And I was back and forth between them both and got pregnant with my son. And I was probably 5 months pregnant when I got with my fiancé now. And have been with him since.

I don’t wanna walk away from everything I have but every time I see my ex I get crazy butterflies and so many memories come back! I need advice y’all!!!