Commitment issues??

I have recently realized I have a hard time staying with a guy for any extended period of time. Whenever I meet and start dating a new person the first few weeks are great but then I notice that I start looking for things that are wrong with them or our relationship and start to distance myself from them. I always have this feeling like there might be someone better for me around the corner or that I am missing out on something by being in a committed relationship. I’m not sure how to get over this feeling. Any advice?