Whats the best way to deal with feelings like this?

like youve hoped and prayed and waited so long for the person you love to return to you and someday somehow things took a twist and what youve been waiting for seems likely to come true. but theyve been with someone else for a long time; then suddenly you feel this horrible pang of guilt for the person theyve been with. its not like youre doing anything to coax them out of the relationship, somehow smth along the way reminded them of you- and maybe you bumped into each other a few times; but its entirely in their hands. sometimes its little things like this that make me want to back away completely despite how much i love this person. like you know stay away from her so she can be happy. since the memories with her bf have already been made and maybe you cant compete with them despite your history tgth. have you ever felt this way?