Guess what this urge will never dissappear. It's man's programming, but it's not that bad you should embrace it and choose integrity and be loyal to your girlfriend as long as you are together. It looks like you are deprived of sex and you have some regrets about your decision. I don't know your situation, but long-term relationships are a difficult thing. Many people make the mistake of staying in too long and miss many opportunities, life is short and love needs time and intimacy to flourish.