How do I stop imagining my ex while with somebody I date or am in a relationship?

It's hard to control where your mind chooses to wander. Your mind "has a mind of its own", if that makes any sense. My best advice would be to make sure you've grieved properly and closed that door all the way.

start thinking about what you like about this new person and why you left the relationship in the first place

If you are imagining your ex when you are with someone else then it means that you aren't fully over your ex and you should probably break up with the person you are dating because imagining your ex while you are with them isn't good and you don't want to lead them on.

Why do you have to stop. Isn't it pretty normal to think random things once in awhile. After enough time passes won't you just stop anyway. Is it harming anything to have these random thoughts?

Accept that there is someone new in your life who loves you greatly and learn to move on from the past

I think you need to assess your feelings. If you like/love that someone you are in a relationship with then it would be easier to stop thinking about your ex but if you can't help but imagine your ex, time to check your feelings.

Forgetting previous relationships is hard. Every relationship has its points that stick out, and you can and probably will remember that for a while. It's important to look to the future and not the past. But at the same time, don't erase your past. It makes you who you are. You will remember sometimes, but it will fade eventually, and remembering doesn't have to be bad. Just remember that it is in the past and work for your future in your current relationship.

After being with someone for a while and them moving on, is hard. I understand that. Seeing your ex in in someone else that you are in a relationship with can be even harder. In my opinion, I don't think you ever really stop imagining past people in the current person. Especially not right away. You see certain traits and qualities that are similar to your ex and that's what makes your mind wander. Try to keep your mind focused on that person. Try talking to the person you're in a relationship more, become closer to them. Maybe that could help. In time you won't see them in someone you date anymore.

its really hard to move on i feel for you and guage you must be feeling overwhelmed with all the memories that you had which trigfers itself whils with someine elae on date..it ia mind boggling and upleasant as you genuinely wish to attend the lady who is with you and care for her presense and her companionship.

Getting over an ex is always hard. Even in my relationship now I think of my ex and what we had. Nothing can take away those memories and you don't have to ignore them. It's okay to remember, think of it and be happy but also remeber that you're with someone new and you're going to make so many more special memories and your ex is in the past now.

You should first understand that it takes time to get over an ex. Some people aren't ready to "get back out there" as quickly as others. Be sure to talk about your feelings with your partner though. It is important that you have a mutual understanding in order to avoid hurt feelings.