My (21M) Old high school sweet heart (22F) who cheated on me made contact with me after 4 years of being blocked, why can’t I get over her?

For a bit more context, I consider this girl to be my first love, let’s call her Sara. I’ve always been madly in love with her and obsessed since the very first time I laid eyes on her. I’m currently in a long distance relationship of almost 4 years with an amazing girl in Tennessee who’ve I’ve know since the 3rd grade, we’ll call her Tasha . She moved from Michigan (Where we’re from/live) to Tennessee with her mom because of work reason. She’s incredibly smart, funny, loyal and pursuing A nursing degree. Genuinely the whole package, I’ve never been too into the idea of marriage but if there’s anyone I’d consider it for it’s her. I dealt with Sara on and off for awhile before rekindling with Tasha, Sara has cheated on me multiple times that I know of and probably more that I don’t, very messy. So I finally decided to cut contact my senior year of school, well like the doormat I am sometimes, she cut contact with me. And over these past few years I’ve done quite well for myself, while it seems she has not, gotten in better shape and started my own masonry business at 21. While she’s a custodian at the school we went to (both of these girls are absolutely gorgeous solid 7-8’s). If you would’ve asked me 2 months ago if I was over Sara I definitely would’ve swore up and down I had no feelings for the girl. But about a week ago she adds me back on instagram and every single positive emotion I’ve had about her flooded back in an instant, it’s honestly embarrassing how fast I folded, she approached it casually and just asked if I wanted to hangout and smoke, and of course I obliged, we’ve hung out about 6 times or so now. I feel helpless when I’m around her, like I have no control over my thoughts and emotions It’s like she has a controller plugged up to me. It’s horrible because I know she’s a pathological liar and manipulator. But I choose not to care, she says she’s changed and she deeply regrets ever doing me how she did and that she’d do anything to get back with me. We’ve hung out quite a few times now, and I feel so guilty about what I’m doing to Tasha. I’m caught in a rock and a hard place, and I genuinely don’t know how to get over this girl. I want to just block her and forget but I can’t bring myself to do it. She knows what makes me tick and just the right things to say. I know I’m being manipulated and played…but she knows id kill just to sit next to her. She’s said multiple times she thinks my obsession with her is hot, I know it’s just a way for her to get a nut off…but it’s a whole lot more for me. I have no one else to vent to because everyone hates this girl lmao and I know what they’ll say. If there’s any other young guys here do yourself a favor and don’t fall in love with a whore.