My partners baby mother still doesn't know I exist.

I have been with my partner for about 8 months. If I'm being honest it really is not the best relationship. Quite frankly, its awful but that isn't the issue today.. I didn't find out he was actually having a baby with his ex girlfriend until 2 months in. By this time I was actually living with him (very soon I know, but the circumstances were all a bit bizarre and I didn't have anywhere else to stay whilst waiting for my new house to be ready.) The fact he was having a bay wasn't really the biggest deal to me, he told me they weren't together long and they got caught, it happens.. Anyway, he told me that he couldn't tell the ex girlfriend about me because she would stop him from seeing and being a part of the child's life. Since then a lot has gone on. The baby is now 5 weeks old. The ex girlfriend STILL isn't allowed to know we are in a relationship and now living together in MY home. She has her suspicions and I don't blame her. The baby has recently been in hospital for a week and he was there constantly. Obviously I didn't have an issue with him being there and spending a lot of time with her because his baby should come first. However he has now made me outright LIE to her and tell her that I just look after his dog for him, he even went as far to say that I am GAY! Which I told her I was not when she rudely messaged me asking me out of the blue on social media. From what I can tell this girl simply wants to make sure her she knows who her child is going to be spending time with and I do not blame her! I would feel the exact same. But whenever I mention the fact I think we should tell her the truth, he hits the roof and tries to make out like I'm stopping him from seeing his child and that its ME causing all the problems. The last thing I want to do is to stop him from seeing his child but I'm now at a lose, what am I supposed to do. When i asked when he planned on telling her he said when she has grown up abit (she is only 18) but he's looking at months/years and I can't go on like that! I have been close to simply telling the girl myself but I don't want it to cause problems for my partner seeing his child.. HELP?!

It's time to get out from under this guy and go and live your life the way you need to. You are in fact making your issue worse by hanging around and continuing to contribute to his deception with his ex. In other words, you're being dragged into it deeper, the longer you hang around with him. Ask yourself why you need a guy who blames you for his problems which are entirely his making and a guy who doesn't have the courage to sort his issues himself.