Am I being used and strung along?

My ex and I have been back in contact for about two weeks now. After being in no contact for about a month. I broke it and I’m the one that cut him off. Well things were going ok until Valentine’s Day when he wished me a Valentine’s Day and then went silent for the whole day. I thought it was sketchy then found out that he was with another girl. He had been saying the things he used to say to me and more all while he’s been talking to her for about 2 months now. He started talking to her in December he claims when we were still in contact and having sex. I found out yesterday he was talking to her after he told me he wasn’t talking to anyone (I know right) well he lives with his mom and she spent the night over his house and she’s posted him on her ig. He told me after I found out that he’s not with either one of us and that they aren’t even together and “just friends”. And that he didn’t tell me because he knew how I would react. And I asked him if they were still talking before and he said no. Why not just tell me the truth in the beginning when I asked, so I could know what the situation was. He doesn’t know I know but she was with him last night and he’s still trying to hide it from me. If I need something he will do it and he wants to see me. But all the things he said he missed doing with me, he seems to be doing them with her now. I don’t know how to feel I’m just hurt. how could he move on a couple of weeks after we broke up. He still answers me when I text and if he likes her more why does he? If he misses me and wants to be in my life like he says. Why is she the one getting his attention rn. We have deep history I’ve known him since I was 16 and he was 15, I’ll be 23 in 2 weeks. Is she a rebound or am I just a side piece now. I don't know anymore. She likes like his other exes and I don’t I was the one ALL of his friends were attracted to. I’m attractive physically, Why won’t he just tell me to move on and stop saying he still has feelings for me.
Am I being used and strung along?
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