Why does my gf do this???:?

im a i have been with my gf 4 10yrs we split in october coz i she told me she was sleeping with guys to get pregnent she has always said tht she is gay and loves me i mean 10yrs a long time to b together well after 1yr and 2mths tryin to concieve she finally has she told me once she gets preg we can b a proper family move bk in and start a fresh (yes she was just usiing man for sperm,we tryed other ways a couple of yrs ago didnt work and very expensive)well now shes a month preg shes told me she has to keep keepin this guy swwet just antil her 3mth scan she told me just txtin him now and again but since this man came on the seen at christmas time she doesnt want to know me she never txtz cnt even spk to me on the fone i thn found out shes been still sleepin with him and now goes round to his house shes been seen in his car (dnt seem like a ''sperm donar'' to me) but wen i confront her im told i'll make her lose the baby with the stress etc shes completely turnin it around on me makin me look bad she tells me im mad to believe it but ive seen things plus i know her inside out being with her 4 10yrs she constanly lies to me i hate her but y do i still love her so much my heart is broken im dying inside y does she do this to me!!