You will hear 'you knew what you signed up' to a lot. The truth is more that you didn't know what you signed up because you (and your partner) didn't give it enough time to get to to know what you were about to sign up and decide whether to do so or not.
It's the same issue once again. Met someone, played happy family at first, enjoying acting like a mum when it's new, then decide that you want to be a mum yourself all this in 18 months. The problem is that at your age, 18 months feels like a relatively reasonable time, but in the scheme of things and as you get older, you realise that 18 months in not even close enough to get to know someone well, build a bond with young kids who have to adjust to separated parents and to experience being a mum yourself for the first time.
You can't rewind time now, so all you can do is try to make the best of your situation and quite a bit of it will be to accept a lot of it. What you don't have to accept though is being another mum to these kids because you're not. Your partner had them with his ex and he is their only parent when they come to see him, so let him get on with it, don't try to impress him and show him what a wonderful mum you'll be too by taking on all the chores of being a mum. The kids won't thank you, why would they, and he won't either because he'll assume you are doing because you enjoy it rather than to please him, which then of course makes it difficult to come out with the truth.
A family member became a SM of 3 children at 18 and had her own child at 19. It worked and 40 years later, she is still happily married with him, so it's not a case of dismissing your age or your situation, but it's not going to be easy and not the dream life you probably imagined it would be when you decided to get pregnant.