Hi and Welcome
Ah Jealousy. I'm sure the moderators will jump in and provide you with the right links so you can read how everyone goes through jealousy with this and it sucks big time.
Unless you are one of a very few lucky individuals who claim to not having a jealous bone in their bodies I think that most of us go through it and maybe monogamy was invented to protect us from the pain.
I'm afraid it's a huge topic but the best place to start is looking at your relationship and your life and seeing where your needs and desires are not being met. Then set about seeing if you can have them met. That for me is Jealousy 101 in a nutshell. If you need clarification just post.