You can access your list of reagents by pressing "J" on the keyboard. In the lower right corner of your screen showing the recipes there is a button that says reagents. It will show you everything you have.
You can also go to the bazaar and click on the sell tab. You can only have 125 pages of reagents. So if that is how many you have then you will want to sell some of the things that you don't need. This will allow you to manage your reagents a little more carefully but I still don't think that will address the core problem. KI really needs to look at a way to ensure that this problem does not occur. At least that we don't loose the rare reagent when harvesting. If you're full on reagents then a message should come up every time you try to harvest that tells you that you are full. In addition, the reagent in question should NOT be harvested. That way someone else could get it while you go sell things to make room. The way it is now, the reagents are going to waste. And with as many people are crafting and the small amount of rare reagents there are in the first place, we don't need any more waste than we have already. Some people will not even realize that they have not been harvesting until much much later and will have taken a bunch of base and rare reagents that could have gone to some one else in the meantime.