I think everyone has bits of your post that they can identify with! I've felt like this before with my current boyfriend. It's hard to keep things new and exciting all of the time, especially when you've been together a year. I think you should break up any routine you have, and do something different. For example, if he always comes round your house on a Saturday, instead do something completely different, like get on a train and go somewhere and spend the day together. As you've only been thinking about this for a few days, I wouldn't do anything drastic, as a year is a long time. Give yourself a bit of time, and then see how you feel. I've found that when I've been feeling like this, I act all different with my boyfriend for a couple of days, like quite annoyed with him, but then it all calms down again. I have these thoughts, but then when I properly think about things, and what it would be like without him, I find that I don't feel so annoyed and I appreciate him a lot more. I also find that when I've been spending a lot of time with a guy I get on well with, even if there's no chemistry or anything, I tend to think about other guys more, which is natural. I know what you mean about missing out on things, but in all honesty do you think you are? A lot of guys out there are not that nice, but you might just not know what you have. However, on the other hand he might not be the one for you, but you just have to think how do you see yourself in a year's time. Do you want to be by yourself, or having more casual relationships, or do you see yourself with him? When you feel like this, talking about the future can be scary so try not to think about that. There's no need to plan for the long term, especially when you are having doubts. Just try and concentrate on what is happening now, and don't think too far ahead.
At the end of the day, it's up to you what you do. I just wanted to say that I understand how you feel. However, as you have been together a year, don't just throw it all away as you may regret it, make sure you've thought it through properly and are 100% sure of what you want to do.