Your feelings about your size may be a contributing factor to your jealousy. But there may also be other issues involved. Really, you are the one who has to answer this question. If you don’t think you are capable of this kind of self-analysis on your own, you might want to see a good therapist. That’s what a good therapist does, he or she helps one come to understand the reasons and root causes for those things in one’s life one would like to change.
In essence, there is something in you that fears your girlfriend will be swept away by another guy who is “better” than you in some respect. What do you think it is? Are you happy with your sexual performance? Do you last long in bed? In my opinion, it is important to us (both men and women) to feel like we can really satisfy the opposite sex (or the same sex, if that’s our thing). Confidence has a lot to do with this. Dick size is only one aspect of this confidence in our sexual performance. But when our confidence in our ability to satisfy our lover is weak, then, yes, it is easy for us to feel jealous, for, once again, we fear that someone better able to satisfy our lover will come along and steal him or her away from us.
The good news is that it sounds like this is largely in your head, based on what you say about your girlfriend’s feelings toward you. Be careful though: Jealousy will kill a relationship. It is good that you are asking these questions. Keep reminding yourself that your girlfriend doesn’t see you in the way that you see yourself. It’s important for you to understand that. Second, figure out what is at the root of your jealousy and your lack of confidence in your relationship with her, even if this means getting help to uncover these issues. Then, work on these issues, whatever they are. Importantly, don’t get into the trap of imposing your lack of confidence on your girlfriend by continually demanding her affirmation of you. If you start to do this, it will cause her to see you in the same way in which you now see yourself. Act as though you are confident toward her, even if you don’t feel this way yet. Trust me, you will come to feel this way about yourself! If dick size is the root, that can be dealt with. You’re at the right spot; your dick will begin to get bigger. And this increase will make you feel in control. If the root is something else, that too can be dealt with. Just find out what it is and go to work on it.
By the way, 5.1” is about average in length. 4” girth may be on the thinner side. But, in my experience, girth is easier to gain. More importantly, most women report that girth is what is most important to them, for obvious reasons. So get going on some girth exercises. Start with the newbie routine, if that’s where you’re at. Then progress from there. In a few months, you could be 1/2” or more thicker, and that will be a very noticeable difference!