I'm a Scorpio too and for the sake of simplicity I'll say I am mono with a poly partner. That is how we started out anyway. While it is still true in practice I feel that I have outgrown that description. I prefer now to say that we have an open relationship and my partner has another love in his life.
A year ago I started a blog www.polyamorouspeople.com
, partly to work through issues myself, but also because everything you have said in your post gets repeated a lot; you are not alone and it is a very big topic. Everything you need should be there, particularly in the early posts.
Just a couple of things though:
1. Don't talk to mono friends about your situation. All they usually do is project their own fears onto your situation and it is totally unhelpful.
2. Jealousy isn't the big deal that you think it's going to be. It usually shows up on top of something else. You fix the 'something else' and the jealously dissolves. For example if your partner is not spending quality time with you but is spending quality time with an OSO, you feel jealous because you aren't
getting your fair share of his attention.
Done badly polyamory can be awful, done well it can be wonderful, but it's a process and it takes time, honesty, love, courage and most importantly a willingness to grow past your comfort zones.