Thank you for reaching out for help. Human relationships are a rocky road that we all have to traverse and though all we need is security and assurance, sometimes these are the most difficult to come by.
Next Steps
Given your history with the person, you need to first be clear about your expectations and how invested you are in something like this. Being involved with multiple partners romantically or sexually is an indication of mental health issues in many cases. The lady you mentioned could be going through something that she has not addressed yet. With that in mind, please have an online session with me or any counselling psychologist of your choice to talk about your feelings and how you can stop yourself from feeling and being hurt emotionally.
Health Tips
Do not blame yourself for any of this and also understand that you need not develop hatred or abusive feelings for anyone because it will only leave you feeling bitter and depressed.