Really need help with break up

Hi everyone,
I have the same problem. I lent my ex girlfriend a lot of money
We breaking up In a very sad way, it was so difficult for us both. She is a really good and nice woman, i had to chose between her and my four kids. If it was good or bad choice, it’s happens. When I’m still process her lost , I saw her with another man. She told me before she will do and she have to go on with her life. She have the full right to do that. 
3 weeks now when I saw her with that guy, feel distorted
, fisically and emotionally . I can’t sleep , I can’t eat , feel deep sadness inside which killing me every moment. I’m not the one who playing poor and showing his emotions easily . So I keep it al inside of me. The biggest problem that I have to passed near her flat very day when I go to my work , no other route!
Seeing the lights on in her flat and imagine what happens their with her new man, killing me every day. Don’t judge me please , I know I deserve it, I need help if you can help me with any advice. When we was together she had finance problems and I help her to solve it. I give her about 4000$ , she promessed me that she will pay it back for sure. I told her she just need to pay me 2000$ . I’m not a rich man , I’m working hard 6 days a week , 10 hours a day to manage my life with 4 kids . She promised me many times that she will pay it back for sure , but I didn’t get any sign about how and when she will pay me back. 
I’m in so horrible situation, I’m already busy to process her lost , then I have to process seeing her with another man. I was writing her a letter ( angry one ) , I need her to hear my heart screaming’ but I didn’t . I just keep silent al the time , beside al of that I saw her happy and don’t want to break that because I still love her so much. I feel so horrible , I saw that side and help me so much to manage my emotions al that time, but now I need real help because I can’t anymore . I feel so down. ( I never show her that). Please help me.
Sorry about my bad English . I wish it’s a bit clear. Thanks in advance.

Unless you take her to court and get a judgement against her she's not going to pay you back. are you posting about the money or her moving on so quick? That happens all the time not much u can do there ..

If she's already promised to pay you back several times and has still done nothing about it, then I think you are going to have consider this a financial loss.

Your other option would be consulting a lawyer to see if there is a way to recoup the money through the court system.