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I'm so sorry to hear things are going this way. You sound like you're feeling a great deal of loneliness and possibly depressed.
I'd highly recommend you seek some individual counseling. Many people find it very helpful when they've experienced repeated failed relationships and are at a place when they are unsure of who they are. A counselor could help guide you through exploring your feelings and actions in previous relationships and what you want in the future. They could also assist in building your self-esteem and re-establishing your identity. Go a few times at least and see if it's something you think could be useful for you. Counseling is completely private and there exactly for reasons such as this.
In addition to seeing someone in counseling, I'd recommend you try to branch out by getting involved some community social activities. There are many clubs, groups, and classes available where you might find a new hobby, meet new people, and keep you busy. You'll distract yourself from some of these negative thoughts all the while adding to who you are through new experiences. Some examples that you may want to look into include a volunteer organization (e.g., Lion's Club or Rotary), parks and recreation classes (they have everything from archery to foreign languages), ballroom dancing, sports leagues / clubs (e.g., tennis, skiing, soccer), art studio offerings, and cooking classes.
I'm concerned about a few of the statements you made... When you say you can't remember the last time you were happy and you're scared to be alone, that's concerning. These sound like depressed statements and I'd encourage you to get help through a mental health professional. If you need to talk with someone at ANY time, please call the following national crisis hotline: 1-800-448-3000
Here's a description of the free service they provide:
Provides short-term crisis intervention and counseling and referrals to local community resources. Counsels on parent-child conflicts, marital and family issues, suicide, pregnancy, runaway youth, physical and sexual abuse, and other issues. Operates 24 hours, seven days a week.
I honestly believe you can feel better about yourself if you work through everything that has happened in a counseling setting. With guidance, you can have a better understanding of who you are, why things have happened the way they have, and how to change directions to live the life you want and feel good about yourself. I wish you the best of luck with all of this! Please feel free to return if you have other questions!