Similar story, same feeling.
I met this girl back in 8th grade, we'll call her Jess, and became friends with her, and just started talking frequently, like daily texting and seeing her in class. Freshman year of high school came around and we kept talking. Come the second semester I started realizing I liked her a lot and she liked me, so we had that awkward freshman pre-relationship shit after being good friends for a year. Around April was the first time I went over her house and we just watched movies and hung out, and hangouts got more frequent, finally started dating in May. It was different at first, my first actual relationship and all that. Towards the second year of our relationship I introduced her to this guy I met at work and who quickly became a really good friend of mine, and we'll call him Tyler. Jess recently started working with me as I worked as a lifeguard and it wasn't really difficult to get that job, it was a big place with over 100 lifeguards.
Fast forward to about 8 months ago, we were stressing out to the max about college and AP testing and all that kind of stuff. Well, me being the dumbass I am tried to convince her to come to the college I had in my mind. So we went with a mutual friend of ours, and she liked the university just as much as I do. We're sitting through this tour and our guide starts explaining her major and what she essentially does. Instantly, I knew that's what I wanted because I had been trying to figure out a major that I would enjoy a lot (Chemical Engineering Major btw). So after the tour I tell Jess how much I really like what our tour guide was talking about and how I think I might try to get into that program. Jess was thinking the same exact thing herself. This is where shit started to go downhill.
A tip for anyone in a high school relationship currently: Don't ever go to the same college as your girlfriend ESPECIALLY the same major. We went about our relationship till this past summer(3 years at this point) and we both got promoted to supervisor at our job and things got pretty hectic. It was hard to make time for each other, we were both clocking in around 9 a.m. and not clocking out till around 7-8 p.m. We started to grow distant, and we could both tell something needed to be done. I loved Jess, and I didn't want to see our relationship crumble just before we got to college, after all college was going to be really cool since we'd be dating and always get to see each other. We both got pretty involved with Tyler, always hanging out altogether and stuff after work, just chilling in the parking lot and grabbing food before we all head home. Towards the middle of summer, Jess and I "took a break" which didn't last long because you can't take a break from someone who you see everyday for about 12 hours. At some point you have to talk and it brings sour memories. We tried to mend things consistently to no avail, but I thought college would save it. It's just the stress from work, right? Wrong.
We got to college and after a few days we started dating again. I'm ecstatic. This new relationship was absolute trash. I was so stressed on making her happy and it was obvious I was having difficulty adjusting to this poorly glued relationship and college as a whole. After maybe like a week she asks to talk back in my dorm after class, and I didn't think anything of it, we always had deep talks in private. So class ends, we're walking back to my dorm and she is quiet as anything, so I think maybe something happened back home and she really just doesn't want to talk about it in public. We get to my dorm and she almost instantly says it's not going to work, she doesn't feel the same anymore and that we're done and there won't be any more attempts. I was devastated. How can someone I've been dating for over 3 years just not "feel the same anymore".
At first, it was mostly just denial. "She'll come back in a week or so". A week passes. "Maybe a month. I bet if I just give her time." A month passes. At this point, things couldn't get any worse for me. I'm at a university, miles away from home, in a single dorm, with no friends, no roommates, and the person I cared for more than myself just dumped me without any other reason than "I don't feel the same anymore". Depression hit brutally hard, I won't get too deep into it, but I'm talking months. Over this time, I'm talking to both Jess and Tyler, telling them what's going on, seeking help from them. Jess gets to a point where all she suggests anymore is a trip to a psychologist, which I didn't want. Tyler was more supportive, telling me how he's been in the same place before under different circumstances and to just keep my head up. I did just that. I came home for a weekend, this was two months after we moved into college, when Tyler was also going to be home. I missed my best friend and he helped me out a lot so I decided to grab dinner with him. We get to dinner and talk about classes and all that stuff, and he asks if I'm seeing anyone. This is maybe a two months after Jess officially broke things off, so I should be doing much better. I tell him I am talking to a few girls, which wasn't a lie, but not talking talking or anything. Just friendly conversations. At this point I still have close to no friends at college. I asked him the same question and he kind of averted it, and I could tell he wasn't really willing to answer. I probed deeper. Started naming names. Of course the first name drop I pulled was "Do you like Jess?" and I said it with a smile, as it would be ironic and he asked "What do you mean like?" and he wouldn't give me a straight answer, so I put it off till after dinner, but boy was I burning to get to the bottom of that question.
After dinner, we get in my car and I waited maybe 5 minutes before confronting him again. He asked if I was doing fine mentally, as Tyler knew everything that's been going on, and of course I said yes. Which was a lie. He told me him and Jess had been dating for a few days now and they had planned to tell me around Christmas time when we were all home and I would be in a good place to tell. In the car, I was happy. I was glad Jess was in good hands, I still cared for her, and yes, still liked her. Not but a week later I started to flop sides on what I thought. I was pissed. And I told him how I was feeling, how everyone is saying how fucked up what he's doing is, and he knew it was going into it. My BEST fucking friend knew he could lose me as a friend entirely and still went on with his choice. That's what got me. How little I must mean to take that risk. He understood how I felt and even admitted he suspected this might happen. I cut all ties with him. I don't talk to Tyler anymore, and I don't talk to Jess. As far as I'm concerned, I wouldn't really care if both of them died.
In the end, I'm a much better person though, I've learned a lot since everything unfolded as it did, and it did change me for the better. I've matured a lot more than ever, and I'm almost glad things happened the way they did because I wouldn't be the kind of guy I am if not. But the overall situation really did suck, as my best friend abandoned me for the girl I loved myself.
TL;DR best friend started dating ex of over 3 years a few months after we broke up knowing it would shatter me