Hey man, I know how you're feeling. I was at that point about 8 months ago. Unfortunately, I wasn't man enough to break it off myself. Until I said I don't do "breaks" and we broke up. The similarities are uncanny.
My ex had some depression issues in the middle of our relationship, which lasted 3.5 years before it ended. I stuck with her through that, until she seemed she was happier. And then I fell into a little slump myself, with the stress of starting a new full-time job, and fear that my mom might lose her job because of layoffs. Plus, the health of my grandparent's was deteriorating. Long story short, she found some new guy at school and they started talking all the time. One night she went to his house for a party, said she would be home around 2am. She didn't get home until 5am, after ignoring some texts from me asking if she was ok. I don't have any proof anything went on between them, but the next day is when shit hit the fan and we broke up for good a couple days later.
Now, I'm not going to tell you what I think you should do. I think you already know what is best, you are just afraid to be single (alone) again. Instead, I am going to give you some insight as to how things will be post-breakup.
First week is going to be tough, you were together for a long time and are used to having someone to talk to every day. Do not reach out to her, do not respond to anything she sends you. Instead, reconnect with old friends you may have neglected, or family members who can support you. When I was going through this, my family and friends really shocked me at how much they supported me. It felt good to know I had them to back me up.
After a month, you will still think about her, but it won't be as bad. You will now think more about the bad times, rather than the good. This is good, as long as you don't act out against her and say some angry things to her. It is best to be mature and keep things civil. You will now realize, however, that you don't need her around to be happy. During this month, you have worked on yourself and are building yourself up as an individual again. You can start dating, but keep things light. Don't rush into a relationship with someone else, rebounds never work.
After 6 months, you take a look back at how you handled things and feel proud of yourself. You have reshaped yourself as a person, and people around you notice. She may try to contact you again, and force herself back into your life. Don't let this happen. Chances are her rebound didn't work out, and she needs confirmation that someone still wants her. This is when you realize you
don't want her. You have had a blast being single the past few months, and are exploring bigger (not literally

) and better things. Life is looking good, and you may actually want to thank her for screwing up so you had an excuse to leave the relationship.
Beyond that, who knows what happens. That is where I am now, and I hope you realize you shouldn't go back to that toxic relationship. You're still young (from what I gathered with the whole underage party thing), you gave a long-term relationship a chance. It didn't work out, now its time to get back into single life and have some fun.
I wish you the best, good luck. If you need anything, feel free to PM me.
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