feelings, emotion, thoughts, mind set

hi,
so ive been struggling with this for quite a while now. i still dont understand why this happens - basically everytime my boyfriend is busy on his games, with his friends anything like that my mind starts thinking about how he doesnt like me enough to hang out with me anymore, or how he doesnt care about me and due to this i end up being annoyed at him and just being moody - but theres only so much moodiness someone can take lol im a little scared he might give up and leave if i dont get myself fixed soon lol - what makes it even more weird is the fact that we have a completely healthy and normal relationship and nothing is wrong in our relationship apart from me just having moods and stuff.
half of mind says that he will leave me if i be so clingy and annoying the other half says that i need to stop thinking stupid things n that nothing bad will happen.
but i dont know how to fix this - its been like this for almost 3 years now.
anyone know anything to help or give any advice please lmk.
thanks x

How often does he spend time with just you? And how much time does he spend with friends?

Sounds like you suffer with separation anxiety an I totally get it. It's put me in some very dark places. Do you know why you feel that way? Has a bad relationship in the past caused you to act out when you feel like you're second best?