I always have the fear that he might leave me

i think that you should talk to him and tell him how you feel and why you feel this way... so all the stress that your having can go away .... if he loves you like he says he does then hes gonna listen to you and paymore attention to your needs.

It sounds like you're going through an emotional rollercoaster. If you've already spoken to him about your fear of him leaving and he's already re-assured you that he has no intention of doing it, it may be a problem of yours and not necessarily a relationship problem.  If this is the case, I would suggest speaking to a counselor/therapist. He/she can better help you sort out where the fears are stemming from and give you a direction to follow to overcome these fears. (For example, Is it possible you have low self-esteem? Or have you been hurt in the past by someone else?)

It's possible that the counselor may also suggest marriage/couples counseling but not until he/she's spoken with you individually to get a better handle on the issue(s) at hand. Good luck!

From what you are saying it is clear that a lot of the problems you have are caused by your feelings as opposed to something your partner actually did or said. Everyone gets insecure sometimes and it's the job of our partners to ensure us that we have nothing to worry about. With that being said, there is a time when our worries become irrational and the worrying itself about these issues starts to cause problems in the relationship. These are problems that weren't there in the first place and were introduced for the first time thanks to this insecurity.

I think that talking to your partner is the best solution but remember  that sometimes the problem can't be solved by the person we live with. Some issues need to be dealt with on a personal level with ourselves in order to make progress. Good luck.

Can you say stage five clinger!  You have to trust this fellow or jump ship, your not only making yourself miserable but your putting him through uneeded stress too.  It's obvious that he already pets and reassures you letting you know that you are loved and he could not leave you so what else is do you think you need? 

One would speculate that you have a serious problem with insecurity.  No one can solve this problem for you and all the petting and reassuring wont make it go away, you have to deal with this or even seek help in the form of a therapist.

GoodKarma is right too, dwelling on the will only manifest your worries that much more.  Give you and your guy a break from the clingy you and show him your independent side, show him that you trust him and believe the things he says to you.  If he has never given you a reason to think otherwise not having trust and not believing in him is a nasty form of disrespect.

Worry is a form of brain diesease, please do not worry until you are sure. Seems like things are normal for both of you so enjoy the moments rather than stressing over something that is not even close.