I am in a one year relationship. What do I do if he keeps constantly suggesting a second girlfriend when I said no? It really hurts me that he would even think that... makes me feel like I'm not enough for him :-(

The whole point of dating is to experience as many people as possible, in order to decide what type of person you want to marry.

Thus, dating more than 1 person at a time is reasonable ... And actually used to be the norm until recently in human history.

So really, people who want monogamous dating relationships are the ones with "problems".

This is a young man that is telling you that you are not enough for him. He will constantly be wanting more. I don't need to waste my time with someone that doesn't value me as I am. You need to decide if your time is worth more than what he is giving you. If this were me, I would heed his suggestion and leave the relationship. He couldn't be more obvious in telling me that I - by myself - am not good enough for him.

But that's just me.