I'm super against the breakup terminology in which you have to have "moved on" from something. Ultimately, this person was in your life for a very long time and impacted it in a, if not positive, at least significant way.
I totally get feeling emotionally overwhelmed from the breakup of a serious relationship and just wanting for the terrible feelings to be over and for you to be "over" something - I've been there 100% and I don't want to negate that that is a very real thing to feel.
The point I'm trying to make is - if you ended up losing a friend that you'd known and loved for the same amount of time, you wouldn't "move on" from that relationship. You would mourn, and process, but ultimately, carry them with you throughout the rest of your life. I think that's okay to do with an ex. It's okay to be like "this is a devastating loss" and to want to not feel that sense of loss so strongly, and I think the best way to do that is to just be like "this was an experience of love that has ultimately shaped me in some part, no matter how small and I will take it with me into new relationships and experiences"
You are made up of love from all the people who have known you. You can be sad about your ex, but you can also miss him! I don't think telling yourself not to feel something ever really works. Let him hang out in the back of your mind, and still know that it's over. You can do both things <3