Totally Unique? Straight, bi, or Gay + Old Fetish :(

(!)Hi, I am in a weird spot in my life and feel stuck. I feel like I cannot safely move in any direction because it could be a mistake.
I am 23 years old, and I've been obsessed with my sexuality for a good while now. I feel like I am unique and one of those people destined to be alone. I have always identified as straiggt but I am not so sure nowadays. I grew up very insecure, low self esteem and basically figured I could never get a girlfriend. Seems I was always stuck in the friend zone at best. I was basically abandoned as a child and ended up being raised by my grandad who was awesome but I was left to my own a lot. I had a few close male friends, I always wanted siblings so I wouldnt be so alone. At about 14 or so I started having sexual fantasies about old men, like over 50. Guess I was into the dad/son thing. I honestly think I just wanted a dad, a regular family but it got sexualized.
Still felt straight all through my teen years. I lost my virginity to a girl at 18 and it was natural, went well, I enjoyed it, and seemed to be a natural. I still masturbated a lot to the old man stuff though, and eventually got around to trying it out. I used to chat with men about meeting, get horny and cum, then lose all interest in meeting. Well, I forced myself not to cum and meet an older guy. I was nauseous and sick to my stomach but went through with it. I was erect but felt funky, and basically just wanted to run. Despite the gut feeling telling me no, I had a few more encounters. All weren't really pleasurable, I came but wast left feeling dirty, anxious and confused.
At 21 I got into my first serious relationship as an adult with a girl I am with now. I feel for this chick hard, and had the best sex with her. I felt good, sexually confident, and maybe even normal. It wasn't long into our sexual relationship my erection wasn't as firm one evening and I got worried. I started thinking maybe I have just "1" sexuality. I always felt I had "2". I had my regular straight orientation, where I naturally went for girls, anticipated sex, marriage, etc. With also, my side sexuality with the old man lust.
I told my girlfriend all about this recently, I have performance anxiety it bed now and basically fear sex. I can get off to regular straight lovemaking fantasies and experience has shown me, in real life old man sex makes me feel sick and confused. Sex with my girl never has made me feel sick and confused. Every time we have sex, I feel worry free. Totally content and calm and at peace.
I told my girl I was afraid I might be gay. I felt she needed to know so I am not dishonest. We have been together almost 2 years and talk of kids and marriage is coming up. I want that life, but I wont pursue it if I'm gay. Sad thing is, I do not want a relationship with an older man or any man. Yes, I could do sexual things with an older man and cum followed by the same sick, nausea that always occurs before and after the act. I have never loved a man like I loved my girl but is that enough. My girl stands by me now, she loves me. I don't want to hurt her, ever.
I just started seeing a psychologist to try and help sort myself out. He thinks I'm straight. Everyone thinks I'm straight or at best Bi. I have an old man fetish, and only certain acts arouse me now. Non invasive stuff. I mostly indulge it watching straight porn. I prefer seeing old men with young women, and I immagine being involved. I used to watch gay porn, but find its not as good as the the straight sex acts. I can imagine being in a threesome with a hot girl and old guy, I wouldnt feel the need to touch him or him touch me. The visual is what I like.
I feel like I'm destined to be alone. I feel straight, then gay, then bi, it never stops. I am in danger of losing my relationship. I am pushing her away, I'm emotionally numb from fear, anxiety, and depression. I don't know what I am or what i should do. Thanks for reading.

 

Hey fellas, thanks for your posts. Sorry for the sorry text job I did, I was literally texting on my phone. I suppose I just don't want to be dishonest, which I haven't now that alls on the table. I think the fantasy is in my case is better than the actual act. I guess I worry that the fact I have the fantasy is an issue. My girlfriend was really understanding, I explained the particulars after I expressed I was questioning my sexuality. She figures It's a fantasy, a festish, no big deal. I guess i am less open minded LOL. I just don't want to be in denial, so I'm trying to get figured out whilst still young so i don't mislead anyone. Thanks again guys.