Why do I let myself feel this way about his child?

I am the type that never cared that he had a child already you know? Like I made friends with the child's mother and know her well and spend a lot of time with the child because I moved in. I watch her a lot while he works.

There are moments when I am so tired from work, and I go home to her and I am just so annoyed. I'm a grown ass adult so I don't act on it nor am I ever hateful towards her because I'm self aware of how I am being extra and she literally hasn't done anything but exist. Shell come towards me and I'm just like.. ugh. I want to be alone, I don't want to be touched, I don't want to hear sounds. But I do it anyways, I feed her and bathe her and incorporate her into everything I do like If I'm cooking I'll invite her to help me and she loves it. I'm being nice even though I'm extremely annoyed. I try very very hard to self regulate and change my emotions and It's so hard.

Then later on I'll cuddle her and watch a Disney movie and I look at her and feel so stupid and so guilty that I ever felt annoyed with her. I love her and she didn't do anything. She's so easy to watch, she's such an easy child. I tell myself I'll do better tomorrow. Why do I let myself feel that way? WHY do I feel that way especially when I'm self aware of how stupid it is? I knew he had a child when I got with him and I knew all of what that entailed and I've never complained or refused to help. I think I'm just very tired, and I'm not used to coming home and having more work to do. I'm learning how to have a child essentially when she's here.

Be easy on me. Like I said I'm never mean to her. We are very close, She's currently sitting next to me with her tablet and I just got done braiding her hair. Please don't attack me. 😩

#1 is this normal?

#2 what can I do to help control my inner emotions towards her? When I get home from work I am so dead, and I try very hard to control it but still feel so annoyed. She's 5 btw I haven't mentioned that.