I will try to be concise about this. I speak from experience, in that I have been exactly as you are right now and how you feel with past relationships. What I have learned is that as prefabsprout said, it is destructive behavior.

That being said, I am glad that you are where you are right now and that is reflecting on this and analyzing it as you are. This is a great place to be because knowing what it is that you feel is off or wrong in your behavior causes us to want to change. As long as you act on this feeling that you know the way you're thinking and behaving (even if only internally right now) is not right, you'll be able to improve.

One thing to think about. Obviously if you act out of these feelings, it will be hurtful to your mate and relationship. Indeed it is already affecting it in a bad way. Not only that, you know all too well that it makes you miserable. Being in love and with someone should not make us miserable and feel tense, stressed or anything like that, no, it should give us feelings of security and comfort. So all jealousy does is throw that out the window. I would assume that on some level you know he cares about you. Thing is that jealousy itself takes that for granted and assumes the opposite. AT the very worst, his time with friends in your mind through the filter of jealousy is saying somehow he is betraying you. find your way back to the reasons you know he cares. ponder and think and force yourself at the hardest moments to argue with jealous thoughts with the truth. Jealousy is rarely based on reality, it's an internal emotion and thought pattern based on lack of trust and feelings of security.

If necessary, get help. find a t to talk about why you feel so insecure and figure out ways to fight it. I say do it now before the wedding or even the engagement or it's not going to be a good relationship going forward.

I express the sense of urgency because frankly I know how bad it can get and you're showing signs of this with your stalking behavior. I know this is a hard thing for you to grasp but driving around to find him, figure out where he is, is stalking, and to me this is a warning sign that you should get help.

Mind you, I hope you know I realize you probably do care deeply for him and I hope the best for you both in your relationship but that is even more reason that I am adamant that you nip this in the bud now, so you can go on to have a great life together

Hope this helps.