I think it's because of your past experiences, you might just be craving physical intimacy. It might not be related to your ex, but just physical intimacy
its obsessive thought, of which u are not in control of.. it happens sometimes. u can distract yourself with activities, goals, friends, talk to therapist. if u r most of the day is going in these thoughts, u may feel more and more anxious and depressive, its exponential. so take steps as soon as possible. and this is nothing related to lust, its a thought and feel which is gone out of ur control. whatsapp if u want more help.
remember feelings are temporary most importantly love in real sense is not feeling it's commitment as feelings are temporary,you know there aren't lacking of things you can look up to quench your desire