Hi jat1221 & welcome.
You have already made a start ~ you have acknowledged your problem. That's good.
Let's look at this objectively:
First try to think why you become jealous.
Is there a reason?
Has anything happened to justify your jealousy?
Is it just around this one girl?
Is it possible that you think that she is so wonderful, that you assume that all other boys will want her, too?
Were you like this for all the 18 months that you were together?
From what you say, she is not encouraging other boys & would still be with you if it were not for the jealousy.
You say about school.
Should I assume that this means that you are both still teenagers?
If so, then this could explain quite a bit, since teenage emotions are complex and run high.
Also, I think boys often find it hard to understand girls, and to communicate with them in ways that help them to understand each other. (This works both ways.)
You say 'her school', so I presume that you are not at school with her. You know that there are boys there, and you are not with her to protect her, or see how boys are behaving towards her, or, indeed, to see how she behaves towards them. This is bound to affect you.
The big question is ~ do you trust her?
If you trust her, then there should be be no need for jealousy.
Jealousy is destructive ~ and it is telling her that you do not trust her.
What to do?
Try to examine what you are feeling & why?
Write down the occasions when you feel or have felt jealous.
By writing these things down, you can sort them out in your mind and look at them in a more detached manner.
What caused them?
Was there any real reason for the jealousy?
How far were your reactions logical & how far were they illogical?
Did the girl respond in ways that fed your jealousy?
Or did she reassure you?
Have you argued over this and made mountains out of molehills?
What do you think she might say, if she were to post on here?
See if you can look at this objectively and find out where things went wrong.
Then try to talk to her.
Try to explain why you were doing what you did and see if there are ways around it.
Long-term, you will need to ensure that this is dealt with, or it could affect any future relationships ~ unless, as you mature, you grow out of it, which is possible.
If you cannot deal with this on your own, then I recommend relationship counselling.
