Fear can manifest as fear of losing the relationship, fear of becoming less important, fear of not being as awesome as the other lover(s), (and I am sure there are others). These fears can be negotiated by simple reassurance and affection; it takes the other person sincerely saying, "yes I love you just as much, yes I want our relationship, yes you are awesome." This fear can go away with time, but it goes away MUCH faster if it is confessed and given a balm of reassurance. It also helps for a person to be proactive in telling the fearful one these things (especially in times of instability).
Loneliness can manifest very easily if people have differing demands on their intimate/connected time; for instance, if person A is only intimate with person B and person B has three other intimate relationships, person A is looking to person B for 100% of their emotional fulfillment, but even if person B is willing to privilege that relationship, they CANNOT give person B 100% of their emotional support while dating others. This one is hardest when moving from an equal ratio (both dating the same number of people) to an unequal one, because the person with less relationships is getting less time and feels this as a loss. Even though the responsibility for working through this emotion rests on the one having it, I feel it is best handled together, because there is a grieving process. It may not be a harmful loss, but it is a loss. Having been in that place, the only way I know to handle the loss of time is to fill my new time with other people and activities; use it to explore, and most of all resist the urge to punish myself for being "unwanted" by removing myself from social things. Even though I knew logically that me getting less time with my lover didn't mean I was less wanted, I still had a tendency to feel that way. How my lover helped me through my loneliness was to give me small pockets of intense connection -- so although I had less time with zir, I didn't feel that I lost intimacy.
Envy is one people get confused with jealousy the most, because it looks the most alike. Envy is when you want someTHING that someone else is getting, but not to exclude others from also having that thing. I feel envy when my lover has an interaction with someone that I want and feel I am lacking -- this rarely happens for me because I tend to be very vocal about my desires for particular kinds of interactions, but with people who are less vocal or less aware of their desires I think it happens more often. For me this one is very easy to fix. I remember once Kyle went thrifting with someone after I had been trying to plan a thrifting outing with Kyle for weeks (with many fails) and I was very envious and hurt about it; after we talked I found out that Kyle had thought that the fails were a lack of interest on my part and the hurt went away, and then we planned an outing (with the understanding that it was a strong desire for both of us) and the envy went away.
Threat (verbiage created by Eanox) is when there is an element that could be destructive to the relationship itself; what makes something a threat varies by the intent of the relationship. With mine, things that create threat are lies, secrets, unwillingness to be civil with me, controlling or manipulative behavior, intense self- or other-destruction, pervasive disrespect, or intense energy consumption (when my lover is with a person who is so in need or unconsciously grasping that they start draining my energy through my lover). I've experienced things that felt like threat which turned out to be misunderstandings, and have only a few times experienced genuine threat. My method of handling feeling threatened is to first of all find out if it IS really threat through talking with my lover and their other person; usually it will go away in this process. And if it is genuine threat, I have to figure out what I need to do to keep myself safe. If a relationship is damaging ME, indirectly or not, I have to put up boundaries which in an extreme case would mean ending my relationship.
There's another element that I've learned looks like jealousy, and that is disconnection. I don't know if other people experience this, because it seems very weird to most when I talk about it. But when I am deeply connected to someone, I feel that connection as a living vessel between us, and I work to keep that energy flowing all the time. Sometimes through upsetness or mental distraction or whatever, that vessel gets pinched closed or the flow slows to a trickle, and I experience this as pain. My reaction to this is to express that I am feeling it and request a moment or two of intense connection to get it flowing freely (or, if necessary, have a discussion to remove whatever is pinching the vessel). Consciously maintaining this connection is something I want for an intimate relationship; if I cannot have it with someone, I do not have a desire to have an intimate relationship with them.
I think it's important that people be able to look at their own emotion and figure out what it really is, because I think that if that feeling of jealousy is conflated with all these other things and the responsibility is assigned solely to the one feeling "jealousy," resentments grow and intimacy decreases. I tend to look at most emotions within a relationship as things to be shared and negotiated, because that turns them from possibilities for person A's growth to possibilities for growing person A AND person B AND the relationship itself.