I'll chine in here...
First, you're 26 years old and I understand where you're coming from because I was there too.
From my late teens into my 20's my first girlfriend cheated on me multiple times. I didn't know what I didn't know and I got caught up in the "Blue-Pill" emotional state. Yes I cried and begged for her back only for her to cheat on me again. I ultimately ended it when she got on my nerves to the point where it just wasn't worth it.
My second girlfriend cheated on me also but It wasn't until later in the relationship. If I'm being completely honest with myself, I played a part in it because I let myself go physically and stopped doing things with her. If I recall correctly, she would tell me to do things with her and I avoided it because I was in the beginning stages of entrepreneurship. All I cared about was trying to make things work for a better financial future. After 5 years together, not only did she leave but got engaged and married another guy within a few short months of a breakup. I was devastated more than you can imagine and that was the major turning point. I was then 26.
At 26 years old going onto 27, I had the same thoughts and feelings as you currently have. I fixed myself up very well and got myself in the best shape of my life and had a heart made of stone.
From the ages of 27 to 36, I was determined to stay single because of my change of perspective. Almost 10 years living the single life, I became one of the biggest players you would ever meet. Enjoying the NYC nightlife and social scene, it wasn't a possibility but a certain guarantee that if I went out on a weekend, I would either end up having a one night stand or acquire a new FWB. I had multiple.
These weren't your average looking Women, they were HOT. My mentality was, If I am in a room full of Women, I will not settle for anything but the best looking one in the room. I had the looks to match my confidence and charisma and was able to pull it off a high percentage of the time.
I became the guy Women were cheating on their partners with.
I am now in a relationship with a Woman who has been after me for 5 years until I gave it a go. Here is my perspective now...
ALL Women, yes I'm generalizing but all Women have the capability to cheat. Will they? this is a complete unknown and you cannot control it however what you can control is yourself. If you ever meet a chick you like and have vetted her thoroughly, you can give an LTR a shot however it is highly recommended to be completely honest and open about things, especially if you are not monogamous. You'd be surprised how receptive Women can be about this but she also needs to view you in a certain way.
I am not deluded into thinking "My chick would never cheat on me" so I'm always on stand by with one foot out the door. if you ever find out there's something wrong going on, no matter how much you care/love a chick or how long you've been with them, you have to be willing to leave. No matter what.
She also needs to know that you are desired by other Women. This is basics 101.
You don't have to fully be against it but be "Red-Pill" aware for lack of better term. LTR's aren't terrible but you've got to be realistic knowing that monogamy isn't realistic either. This is the core of the issues people have. will he/she fvck someone else behind my back.
As Men, we know what other Men are thinking if they see you're GF and she's attractive. If your GF knows this too and she's talking to other guy "Friends", you know you need to cut the rope. The only way an LTR works if she respects you and believes you will cut the rope if you suspect any wrong doing.
You can opt to choose to be a single bachelor, there's nothing wrong with that either but sometimes it's favorable to have a GF who cooks for you, runs errands for you and do things with. Will my LTR last forever? I don't know what will happen but I can tell you this, I'm not blind to bullsh!t anymore because I've been with so many Women that I can sniff BS a MILE! away. Comes with experience and heartbreak.
One of the pitfalls many Men fall into with Women is getting into an LTR for Sex. Maybe she bounces off your d!ck just the way you like it, maybe she sucks your c*ck so good you experience fantastic orgasms. Whatever it is, but I can assure you, there's millions of other Women who can do the same if not better.
You don't have to be against LTR's but you do have to be self aware and not let emotions get in the way of your judgement.