I'm bisexual, but I don't know if I want to be with the guy I've loved or the girl I'm in love with?

About a year and 5 months ago, I met the guy I am presently engaged to. We met through the death of my brother, his mom helped out with the fundraisers and everything. We've been together off and on, and this time is the 3rd time we've been together. We've been though a lot of mess, running back and forth, him being indecisive a sexual friendship type ordeal but never the less always coming back to me. He didn't actually come back and ask me to marry him until after, he'd seen me with my ex girlfriend. Her and I were together for 3 months, and it was literally the best 3 months of my life. It was the first actual girl on girl relationship I'd been in. She was my best friend, and lover all wrapped up in one. Just as he is, but however I got the love and affection from her and want that I'd never been shown by anyone. But I'd ended up leaving her for him, and then realized how much I really missed/loved her. She says it's just because I was in love with the idea of someone really wanting me. Which was a lie, I knew I loved her when we were together. I just didn't know what I wanted when he came back and I already had feelings for him and I was on the peak of knowing I was really in love with her... She went and got with someone who is like, 8 yrs younger than her. She's 22 and the girl is 15. Which was a friend of mine, she says it's too late for me to realize I love her because now her and the girl have been together 5 months. But she questions it, we had an argument one day, and I said what I had to say about me loving her and it was like well maybe idk idk.
Now in the first half of their relationship, her and I were sneaking to see each other and to talk to each other. She was still in love with me. And even now, she talks smack about my fiance and is still sneaking to talk to me when her it thing isn't around. Which isn't often, but I really miss her a lot even though I'm happy where I am. I'd take her back on the right grounds, I'd love for us to. But I don't know about her. It seems she's unhappy in her predicament, and I don't get why she still has to bring up our past sometimes and talks crap about my fiance about our sex life and she was the best and she doesn't even have sex now which is odd, cause we were like 2 jack rabbits. I love him a lot, but there's something about me not even the same without her. A feeling I don't have anymore, and I worry so much more now that I'm with him and freak out. I never did these questioning over analyzing things with her...The little girl she's dating is only in high school, and she's trying to be a cop and talking about wanting to be with someone who can put in as much as her into a relationship. Being a team with someone..the 15 yr old has had a bad rough life in and out of state custody for not going to school and VERY mentally troubled. Do I try and stick out my relationship with someone who is also 7 yrs younger than me, but he loves me or try and see if I can get my girl back? I have no clue what to do based on all these factors..