I get jealous and possessive of my romantic partners' or friends' time when they spend it with other people (that aren't me).

This is due to egocentricity. I do it myself at times. If someone is ahead of me in a queue, I can become annoyed with them because I assume that my own needs are more important than theirs.

We are not inherently more important than anyone else; none of us. If you have a friend who is spending time with someone else, said friend is perfectly entitled to do that. Distracting yourself temporarily is not going to solve the problem. Realising that being Scorpio does not make you inherently more special than anyone else, will. Having a vile, sadistic temper does not imply greater competence or intelligence, and it certainly does not imply greater compassion.

Be aware of something else, as well. The mature Scorpionic state is sometimes referred to as a reborn phoenix. While said term implies the level of grandiosity that is customary for Scorp, there is an important clue here. Namely, that the only reason why Scorpionic individuals decide to clean up their act developmentally and cease being psychopathic, is because their bad behaviour will cause a backlash to the point where the Scorpio native will almost end up dying, and will therefore have to restart from scratch. This is the entire reason why said phoenix-like rebirth ability is necessary in the first place. You usually end up using it to atone for your past sins.

So what I am offering you is the opportunity to save yourself some time, and potentially a future trip to the hospital, here. Scorpio has two choices. Either it can wait until its' usual attitude nearly destroys it, or it can recognise its' arrogance and sadism, and attempt to purge those from itself, before life attempts to physically beat said characteristics out of it.

Something which the other signs inherently know, but which Scorp individuals can be a bit slow to figure out at times, is that the entire reason why immoral behaviour is a bad idea, is because it is ultimately self destructive. Scorpio tends to think that for as long as it can get away with it, acting like Hannibal Lector is just fine; but the problem is that you never do get away with it forever. Eventually, universal law asserts itself, and you get what has been coming to you.

So put the sting away, (I'd actually recommend amputating it to be honest) and focus on being kind to people. While sadism might feel good in the short term, ultimately you are the only person who will be hurt by it, if you choose to indulge it. Remember Google's previous corporate motto; Don't be evil.