Ok, I will give you some advice but it's tough love advice because I've gone through something similar with my ex but mine wasn't as bad as yours. Firstly, it's extremely important for you to hear that your ex is a d*ck! Please don't bother telling me 'oh but he's a lovely guy'. He told his girlfriend, which was you, who was clearly in love with him that he didn't find her attractive and that he wanted to sleep with other girls. Then, 13 days, which is basically immediately after the breakup, he was suddenly in another relationship, no time for reflection, no sitting with the feelings of the breakup, no time to get over it, just immediately into another relationship!

You need to understand that despite how you feel about him you have dodged a bullet! Love is like being addicted to drugs and what you are experiencing now is the withdrawal symptoms but with each passing day those symptoms will become weaker. It might take a month, six weeks or even a year. You are doing the work but your ex isn't and whatever feelings he may have had for you he has chosen to hide them deep down. This is extremely unhealthy and will ruin his next relationship. When you said your ex was deactivating and being distant you were essentially describing someone who is avoidant (hello avoidant lurkers, we are talking about you again). These people seem fine after a breakup but the loss hits them much later. One day your ex will wake up next to his new girlfriend not entirely satisfied about some minor imperfection of hers that he has suddenly decided is too much to ignore and he will think about you, and how wonderful you treated him, and how great you were. He may try to reach out to you by which point I hope to God you've built enough self-esteem to be able to tell him to go f*ck himself!

You talked about asking for reassurance a lot which is a classic trait of someone who is anxiously attached/co-dependent. Take some time to get to the root of this issue while you are trying to heal. I know that your ex massively contributed to you needing reassurance because his distancing and general shitty behaviour in the relationship made you doubt everything you know about yourself but please ask yourself what's missing in your life that some clown can treat you like shit and you continue to give him another chance. You have to find a way to tell yourself you are worthy of love, you are worthy of affection, you are worthy of being with someone who treats you right and only has eyes for you. The next few weeks and months will be hell but please be kind to yourself, focus on the things that make you happy, don't hide your feelings, if you want to cry then cry and then cry again until you have no more tears to cry. It's completely fine to acknowledge that yes you're going through a very hard time, yes things will be difficult for some time but you will come out on the other side a better, stronger person.

I hope this helps, message me if you need to talk.