Go easy on yourself!
I see a lot of self criticism in your post. You got swept up in a romance - this happens! Maybe better to be open hearted and and hopeful than cynical and bitter.
It’s ok to feel sad about a losing a relationship even a short one. Sometimes they are they are harder to let go as the Idealistic fantasy is still so strong with less of the actual reality of the person. So you can imagine them to be whatever you want rather than what they actually are. I think also loss brings back echos of other loss and. It sounds like your long term relationship ended in a fairly traumatic way so you may also be feeling reverberations of that.
My advice for getting through it -
Have you got any friends apps dating? Talk to them, laugh with them, commiserate with them. Really helps to share the experience with other people doing it (rather than well meaning long term married friends). Nearly everyone on the dating apps will have had an experience like this so will get it.
Find the gifts - look for the parts of this experience you can take away as a positive. Maybe that’s lessons you’ve learnt about yourself and your ability to connect and love again. Or experiences you had that you wouldn’t take back. All of dating is data about you and there is a some good data in this experience for you to use going forward
There are lots of good podcasts about dating with really helpful ideas about all stages and heartbeak. Some examples - do the work, Jillian on love, Open house with Louise, it’s not you it’s them. Dig around and see if you can find someone that speaks to you.
Abundance over scarcity mindset - how amazing you found someone you connected with so quickly. There will be more when you are ready . This man quickly showed you he wasn’t it so you can now see what else might be right for you. It won’t be the same but it may be better.
Try and find fun in the process of dating. Don’t fixate to much on an end result which will lead to frustration and disappointment
As others have said think about the things you like about your life and sink into them. Anything that helps you feel good about yourself and your life. But, also don’t try and fight the sadness if it’s there. You need to feel that and go through it. To get the highs from love, connection and romance we sadly open ourselves to the lows as well.