I'm wondering how big of a role fear of rejection plays in your relationships and what it stems from.
I have no fear as such, if I get rejected by someone I care about in some capacity, then it surely doesn't feel good, and I don't like it, but I do not place fear of potential/future rejection at the front end of my decisions as to how to interact with people. I get over things quickly, I always have, my Mother hated it that I was so forgiving. I do learn from my mistakes though, and if rejection occurrs and is hurtful, I hold the other person accountable for their actions and will not give them the same place in my life until they have apologized, demonstrated sincere regret, and make an effort to earn my trust back over time. People fuck up, I have before and been forgiven and it was a huge relief, but there is a right way to do it, and you can't let people walk all over you. Sometimes you have to tell people to fuck off, the effort entailed in restoring things is too great and too much a bother, but you don't know until you try to figure all that out. Life can be very hard sometimes in this capacity, and the people that can hurt you most are those that are closest to you. What a cluster fuck!
NTs are notorious for withholding their deeper feelings from others and can take a long time to truly open up but shut down in a matter of seconds if the other persons reaction is unexpected or unwanted.
SPs are not, but we are human and can get miffed if treated the wrong way.
So what does it take for someone to get you to open up?
I open up a good bit on my own, just out of being me, but I do of course hold back my important personal details until a good amount of trust is established. For them to demonstrate to me that they like me, appreciate me, understand me, and accept me as I am, that will get me to open up.
What role does fear of rejection play in your relationships (not just romantic ones)?
None. I set out to do what I need to, if rejection happens I move on. Don't like my idea? Your loss. Don't want to go to the prom with me? Your loss. I have high self esteem, and I know what I have to offer.
And how is this linked to your self image?
Self image =

I'm good to go!