Relationship advice?

I have been with the same guy for a little over 2 years. We broke up once before about 8 months ago and were broken up for about a month. I heard he was dating a new girl and it felt like someone had shot me in the heart. I was focused on getting over him and bettering myself that I didn't think about him seeing someone else.   I love him so so much but I'm genuinely unhappy in our relationship. I know I should break up with him but I can't fathom the feeling of seeing him with someone else like I had before so I've been staying with him. I've felt so stuck since I realized that's the only reason we're together. I want to move on and better my life and become a happier person but I really don't think I can handle seeing him with someone else and knowing he is in love with someone else. Idk it probably sounds stupid, I just need help. How do I get over this?