To think that you would be in a relationship for any reason other than it makes you happy sounds dumb when it crosses your mind. You think, "If I'm unhappy, why wouldn't I just leave?" But sometimes it gets hard, especially the longer the relationship has lasted. You start to overthink everything.
Your family knows them — like, your whole family. Grandparents, distant cousins you only see during the holidays, the whole shebang. They like them. They think you're happy with them, and you don't want to let them down. Not only that, but you'll have to have that awkward conversation the next time you see each of them and break the news.
Things get complex the older you get. Your lives intertwine a lot more than just in high school where you'd try to hang out after practice or find a way to walk with each other to class. You start to plan your life around their life, and vice versa. You don't want to disrupt the pattern, the flow, and the way of life you've become accustomed to.
You become more focused on making other people happy with your love life, whether that's friends who have looked up to your relationship, their friends that you've grown to like, or even just your parents. Your life isn't even really your life anymore at that point — it's everyone else's.
You need to live your life for yourself, and that includes your relationships as well. If you're unhappy with the love situation you're in, you need to leave it. You don't need to justify your decisions to anyone other than yourself. You don't need to prove yourself to anyone but yourself.
You deserve to be happy, whether that's with someone else or just being single and finding out who you are alone. Your life is your life, no one else's, and your love life will improve immensely when you are genuinely happy with who you're with, or lack thereof.