Hi, I am exactly in the same situation as you. Having a 5 year relationship with my first bf, and think already for more than a year that I might not 'love' him anymore. I say 'love', because I am quit sure I love him with all my heart, that's what it makes it so difficult, but I don't know if I am still 'in love' with him, like what's necessary to stay longer together.. I am having a hard time finding out, and also me I am afraid to regret it afterwards. I am quit sure I will miss him a lot, but maybe it's the best decision... I met one other guy, for whom I immediately felt somthing I've never felt before, and I just really want to try it, also afraid to stick to the first one which is good, but maybe not the best...
I know it's difficult, but should also say, follow your heart and feelings, you will only learn from 'wrong' or 'not completely right' relationships, and every time you know what you are not searching for in someone, and it will be clearer and easier to find 'the one'.
Good luck!