How to find meaning after a breakup?

I had something with a girl I really liked, and around 6 months ago things ended. I made the mistake this girl a big part of my life. Doing everything with most days of the week. Unfortunately, she decided she wanted to go back to her ex. She never told me anything and just had me on the side until I found out by other means. I cut her off and I haven’t talked to her since, and I am not meaning to do so. The problem is that since then I have struggled to find meaning. I go to the gym, work, read and try to do things to improve myself. However I can’t help but feel empty. Sometimes it seems like it goes away, but then just suddenly one day I feel bad again. Most of our common friends were more her friends, so since then I haven’t talked to them. I don’t really go out anymore for anything besides the necessary or the gym. I can’t help but feel like a loser sometimes. Knowing she is doing well and that I was left with the aftermath trying to find out how to feel full again. And right now, I don’t feel like I even wanna try again anytime soon. Opening up to someone now seems like something full of disadvantages and disappointment. I just wanna feel normal again and forget about all of this. What advice do you have to get over this? To stop feeling like a loser and eventually forget about her, and move on.