You don't need to leave! As the previous poster said you do need boundaries. A date night isn't unreasonable. She's 13 in 3 years she wont even want to spend so much time with her dad and in 5 years she will be going to university.
My husband has a daughter and like you I found it hard. I've never wanted children or felt I even liked children so it was hard for me. I had feelings of jealousy too. I got with her dad when she was 7.
It was hard and I talked it through with my husband who understood my feelings. We worked through my issues and his daughter never knew I had any. She is an amazing girl and I actually adore her, but resentment took a while to get over.
You sounds like you know its you with the problem, like I did. If you've self awareness you can sort it out.
I'm very close to his daughter now. We get on very well and always have. She has never known the issues in my head at the beginning as her father (my husband) knew and made time for me too. Time as a couple is very important.
Talk to your partner. There's no shame in these feelings the only shame is if you act upon them. It sounds as if you dong and your stepdaughter enjoys being with you.
When lockdown is over make sure you all do activities together. I play a certain sport and I've managed to get his daughter into it too. When she is with us we do that and that means we both get to fo something we enjoy. It also has brought us closer together.
Also I'll go out drinking with friends to give them alone time, but really its s chance to escape. 
There's ways around it. Just be honest with your partner. If he's pragmatic he'll work with you to encourage your relationship with him and her.