Hey I just went through the exact same thing 2 months ago when my girlfriend and I broke up after being together for almost 4 years. She was becoming more and more distant, and would do things like "be hanging out with her friends" and not come home until the early morning hours. And then my dog died and instead of wanting to be with me that day she wanted to go out with her friends. She never admitted to cheating, but it was clear she was done, and we broke up right before Christmas. The following days and weeks were incredibly rough and I was an absolute mess. It was around this time I found this site and learned about SN, but it would unfortunately be awhile before I could get my hands on it. So in the meantime I tried to make life as painless as possible. I got high, played videogames, watched tv/movies, ate a ton of junk food, and spent a lot of time lying in bed buried under blankets. Now it's been about two months, and it still hurts, a lot. It's even worse seeing how quickly she moved on and how happy she is now. But, it doesn't hurt as much as it did initially. I can now look back at the relationship and my life with more of a clear head. Those first few weeks were chaotic, and I wasn't in a place to make any long term decisions.
Im not telling you what to do, but personally I'm glad I didn't have access to SN right after the breakup and had some time to process it. I very possibly might still use it, but I'd much rather make that decision with a clearer head and be confident im making the right decision. A major breakup is like a death, possibly even worse because they are still there but they aren't with you. You go through the five stages of grief, in a random order and repeat stages often. But over time, those intense emotions subside. They don't go away, but they become much less intense and overwhelming, and easier to understand. And I think that will become more true with more time. My depression is still there, and the loneliness is often unbearable, and I'm tired of being in pain, but if I decide to ctb I don't want it to be a spur of the moment decision. I want it to be in a place mentally where I'm at peace with that decision and confident it's the best option.
But the pain you're feeling now is a lot to deal with, and I know nothing can fix that right now. But me and the members of the forum are here to listen and be supportive.