I can relate to you in a way, my BF his ex appeared after 8yrs and told him he had a daughter, and they ended up having an affair and she told him she did a pregnancy test the day after they had sex and she was pregnant to him again...i'm a sucker for believing in him, and his sorry's and he wants me not her etc.....the women then told him she was wrong the daughter isn't his and the baby died, when she realised he was still there for me oh man and we're in our 30s...anyway long story short...I gave him many chances, and tried to be understanding about his daughter, he needs to go through paternity to finalise this mess his ex created.
I just cant stand it, I too get funked up over it..i'm jealous and I feel selfish, i'll admit that, I don't want him to have a daughter to someone else, I don't want him to go for paternity tests, I want him to believe his ex when she says the daughter isn't his, so we can both move forward for our future and make our baby like we planned together until his ex appeared then he pushed my wanting his baby away, he spends more time worrying about a paternity and less time worrying about making our baby that we'd been trying for 2yrs...I feel like if his paternity test is positive then what would he need me and a child with me for, that's just my opinion anyway.

Just a thought, how does your BF feel about you? He still wants you? and paying child support is a good thing