Why / How do I stop being so jealous?

The short of it is, I have had 2 major relationships in my life.

One was my High-school sweetheart and bf of 3 years before we dated for another 4 years. She wound up cheating on me while I was away at basic training in the army.

The second was a girl I meet a few years after the I left the army and we had dated / lived together for 2 years before she also cheated on me.

After both relationships I asked both of them if it was something I did to drive them away so that I could understand the problem better and hopefully better myself so that it wouldn't happen a 3rd time.

My High-school ex said it was because we couldnt connect on an intellectual level (She was going to college for pre-med and all I have ever wanted to do was be a police officer.) I will admit we did have a gap between us in that department. She was a 4.1 or 2 honor student in High-school and revived a scholarship to college where I was a 3.0 and joined the army so my parents wouldnt have to carry the burden of my college bills.

The second relationship was a bit different. She basically told me that she had just grown bored and didnt care about our relationship anymore but at the same time didnt want to break it off with me because she was afraid of hurting me.

Well after both these relationships (I am now 26) I have found myself interested in another girl (it has been about 3 years from my second serious relationship) but I am noticing that I have become jealous for no reason. This current girl and I are not dating but have both voiced interest in each other and we are taking somewhat slow so we can get to know each other a bit better before we commit to anything.

Well today when I was speaking to my best friend (a male) on the phone he told me that she gave him her number to speak to him about a project all 3 of us are working on. When I heard this I will admit I got a bit jealous of the fact she did that even though I know for a fact there was no other intent behind her giving him, her number other than the project. This kind of scared me because like I said we are not dating and she as far as I know has no interest in him in that way, and even if she did I feel I would have or should have no right to get upset at this because right now we are pretty much two people who "have a crush" on each other.

Please help me with this or at least point me to a source in which I can get some sort of help to nip this problem in the bud before it becomes something worse.

Any advice is worth commenting with because it will at least give me different points of view on this problem of mine and may help me to solve it myself.

Thank you for taking the time to read and respond to my wall of text.