Why is my ex boyfriend acting like this?

So, I'm gonna try to make this make sense and I hope this is the right place for it. My ex boyfriend (35m) and I (38f) were together for close to 9 years. We ended up drifting apart about 1.5 years ago. We never formally broke up but remained friends. I was going through some difficult problems in my life with my mental health. I ended up in a psychiatric hospital due to my borderline personality disorder I was diagnosed with 15 years ago. I never really got treatment for it because I was in denial anything was wrong. We kind of went our separate ways but stayed good friends. He advised me to seek treatment and to get better so that we could resume our relationship once I was stable. I took therapy and medication seriously. I got better. He praised me the whole time. Once I felt ready, I told him I wanted to try things again and he hit me with a bombshell by saying he had been seeing someone for the whole 8 months we were apart. I was so hurt but mostly because he didn't tell me he was talking to someone. I was also hurt because I wanted to get better for him and for me. (Which I did so I don't regret that.) Here's where it gets weird. He said a girl at work just kind of "pushed a relationship on him." I guess she just called them boyfriend and girlfriend and he went with it. He's kind of quiet and laid back and never really had a voice for himself. I was actually his first girlfriend and he didn't have any experience with dating. I asked him if he had feelings for the girl and if he loved her and all he could say was "She's a nice girl and she cares about me and is really attached to me." I respected his relationship and choice. Then things got weird.

So he said he still wanted to be friends and cared a lot for me and that his feelings for me have never gone away. He said he can't break up with this girl because she hasn't done anything wrong to him or to make him want to break up with her. So he's basically with her because he doesn't want to hurt her feelings or have any bad karma come his way as he puts it. We're only platonic friends but we text every day. He only sees her on Fridays and Saturdays for a couple hours. She doesn't stay the night with him. They don't live together. He's never met her family. I told him his relationship sounds more like a friendship and he just said yea it does. He told me he has zero plans to marry her or anything. So far they've been together for a year like this.

Sometimes I notice that he will text me vague messages that confuse the hell out of me. He will constantly text me to vent or tell me about his day and problems. He's constantly negative saying he hates his life and job. I told him he should be happy since he has a great girl who cares about him and he gets mad. I wasn't saying it to be a jerk. I was just trying to cheer him up. Sometimes he tells me about how he wants to try things again if he ever becomes single and I told him I'd be ok with that if it's meant to be. He's even told me how much he thinks about me and dreams about me. He says I still turn him on. (We are not intimate in any way. I respect his relationship.) He says his feelings for me have never changed. When our conversations turn to that, he will ask me if I still think about him and when I say yes he immediately backs off and says he's in a relationship and can't talk to me like that because its not appropriate. I don't engage in it too much because I can't ever tell if he's single or still with her. Sometimes he makes it sound like he wants out of his relationship and then other times he acts like he's so happy. I'm just so confused by him. One time he made a weird comment when we met up in person and said "I'm just so conflicted. I don't know what I want." A couple weeks ago he took me out to lunch for my birthday and paid for my food even though I told him I would pay for mine. He texted me happy valentines day. But he is 100% still with this girl. I know we are just friends and I don't try to overstep my bounds, but his mixed up mind has me so confused. What do you guys think? Is he just manipulating me? I'm trying so hard not to spilt and lose it with my mental health but he makes it so difficult for me but yet at the same time, he's a support system for me because I literally don't have anyone else in my life. Please help me understand this weird behavior. I'm just so confused as to why a man who is in a supposed happy relationship with a great girl would act this way with his ex? I know I should just stop texting him, but we were together for so long and I don't know how to let go of my best friend.