Don't take it to heart. As other's have said, she's only 2. Give it time.
We have been a blended family for 10 years and between us we have 6 children. It's been one hell of a ride so far and I dare say it will continue to throw us curve balls. When we first got together one of the middle girls said to her Mum "You're not my Mum any more, I've got a new one". She was 8 and out to hurt. I have no time at all for her Mum but I was horrified. 3 years ago the oldest SD told me that she hated me, I wasn't her Mother and to stop acting like it. She was also out to hurt. She succeeded. She was 18 at the time. This is also the girl that has called me the "best Step Mother ever". Go figure.
Bottom line is sometimes you'll be loved by your step children, sometimes you'll be hated. Sometimes nothing you do is right. You are the bad guy, the fall guy, the source of everything that has gone wrong in their lives, and their parents lives and the world in general.
As a step parent, you need to have an open heart, broad shoulders and very thick skin... and I'm afraid you WILL always be third, and sometimes, you will will be rejected. It's just one of the joys of being a Step Parent. She has a biological Dad. That Dad can be a complete looser but she will still love him unconditionally and he will still be her Dad. BUT, you can be better. You can be the one man in her life who is always there, rock solid and reliable. In the background sometimes, but always there. The one man that, as she grows older, she will know that if she needs you, you'll be there, no questions asked.
My ex was in some ways a lovely man, but he was also unreliable and childlike, his humour was often inappropriate, he could be really embarrassing and infuriating and in many ways he wasn't part of my kids lives - but he was their Dad and they loved him regardless. When he passed away suddenly last year they were devastated. BUT my DP, who has raised them as his own for the last 10 years, and who they struggled to accept for a long time, continued to be amazing, and has helped them through.
They loved their Dad, but they came to love my DP too. He has been their rock for the past 10 years and will continue to be. When they need help, from fixing a broken toy when they were little to being picked up in the wee small hours of the morning after a night on the town, no questions asked, they rang him, not their Dad. He was the one that taught them to ride a bike, then to drive a car. He got my son his first job. He was the one that gave the boys "that" talk when they were old enough and was there the first time they got their heart broken. He's not perfect and he's not their "Dad", but they love him for him, and now that they are older, they are beginning, just, to realise and appreciate just how much he's done for them.
For you, give it time. Lots and lots of time, and patience, and love, and understanding. Yes it's hard work, and you are going to have to learn to take a lot of stuff on the chin, but it's worth it.
As for struggling to love her, of course you are. Just because you love her Mother doesn't mean you will automatically love her, in fact it would be weird if you did. She's not your blood. You may COME to love her, but you may not. All you can do is try, and give it time.
Good luck!