If the stripe is tape use a hair drier on a warm day use your fingernail to lift one end of the stripe after warming it with the drier and then warm the stripe as you slowly peal it off. If it is painted on DO NOT try to remove it yourself. By asking this question it shows you do not have the knowledge or skill to do the job. If you try you will probably damage a very expensive paintjob Also do not listen to a well meaning friend about buffing it off ithout experience you'll most likely go through the stripe as well as the actual color on the car, again VERY expensive to fix
Look sometimes people just change or grow in different direction. IF you still love her and want her back STOP BEING A SCHMUCK If you are one at all, Do not complain. Do not argue, BE NICE no matter what! Act like you are not upset or bothered because that only justifies cheating in her mind.
IF you are wrong and she is not cheating, try joining in with her on a new hobby try golf, boating, bowling, pool, anything you both talked of or used to do. Take a class or a trip together to someplace fun, not romantic because having fun together will lead to romance and she will not feel all you wanted was sex. Re enact the 1st date you had or the best date you had or if she has a non sexual fantasy like going up in a balloon or going to some exotic place, make it come true. Be an active part of her life not just a guy she comes home to and argues with all the time or humps once every 3 months.
IF she is cheating, there are lots of reasons people cheat. they could be morally bankrupt or just bored or seeing something they were not getting from who they married. The reason does not matter except to you as education so if you became boring, non caring smelly or whatever the reason, that you can see it and change what you did IF it was you and it may very well not have been. It could also be simply that new is exciting and she wanted to feel that excitement or it could be money. My ex married my bosses son after starting her affair with him while I was in the army. I was friends with his other brother and for that reason we saw him around quite a bit while we were dating, and she used to say how fat stupid ugly and crude he was and how she could not understand why his girlfriend would go out with him. One day the fact he had a ton of money from his father business and would inherit it she zoomed in on him like a rat on cheese. Pops died fat boy got rich, fat boy lost it all, she divorced him and married an 84 year old guy for his money. My advice is look at what happened to you deeply. If it was your fault even a little, learn from your mistakes and forget her. Move on and be happy. It's not easy but can be done and trust me there are a lot of women out there it just takes time but beware scammers who will try to zero in on you and drain you financially. If you don't find a woman here look overseas you can find younger beautiful women who will treat you like a king and honestly love you BUT AGAIN beware of scammers who also live overseas NEVER EVER SEND MONEY to anyone regardless of their hard luck story. The main thing is to get back into dating as soon as you can or at least to find a woman to write to. Just don't cry about your ex or even talk about her. Just be a good guy and treat any woman you meet with respect ansd kindness. do not use them and do not allow them to use you and put your past behind you. Find some hobbies where you may meet a woman, anything from golf to bowling can help you have fun and meet people who enjoy the same hobbies you like. Just have fun and don't let her get to you.
Good luck