I've been grappling with both lately. I'm trying to coax them into development without being forceful.
I don't """feel""" Si very often in an active sense, it's normally a sense of apprehension before anything. I've seen Si described as a personalized roadmap, and mine isn't very intricate other than a few fundamentals. It's a thing that tugs from the back of the mind, and when I actually let it "speak up" it's often comforting. It feels like a certain stability that gives me rewarding feelings when I indulge it - a near-smugness that I averted/avoided something negative by listening to it. Sometimes I don't understand it though, like it's my teenager I can't see eye-to-eye with. Why must I feel compelled to hoard NatGeo's if I hate rereading things? Why must I keep indulging in idyllic versions of memories when they only weaken my already tenuous grip on the present?
I've also noticed it becoming more active though, such as my sudden urge to try as many varieties of wine as I can and write down my personal impressions of each. A year ago this would've sounded boring to me, at least the recording part. I'm finding delight in comparing new impressions against the old. "Ah, yes, this merlot tastes like merlot, as did the other, but it is much more 'cherry-wood den' and less 'hooker with a heart of gold." As an added bonus I can engage Ne to come up with possible descriptors, and Ti to use this information to fatten up my understanding of the universal Wine and how it incarnates.
Fe is more like my ESFJ friend - when it works, it's exhilarating and fulfilling, but when it doesn't I want to pull my hair out and punch myself in the face for thinking we're meant to be in the same room as each other. On the one hand it can be really fun when it works well. I get a rush when I make someone genuinely light up by asking a question about their life others might miss - like "I remember last time you said your aunt got in an accident, is she doing better?" On the other hand I don't like that I can get so butthurt about people neglecting/imposing on my emotional boundaries when I know I can't logically expect people to just psychically be aware of them. I know I can look pretty childish when I throw a fit just because I feel like the people that are better at socializing are rubbing it in my face, when really they're just being them. I also get very restless if I don't "feed" it occasionally. Even though I'm more introverted than my other IXXX friends, they could go almost forever without interaction, whereas I NEED interaction every so often or I'll die, even if my socializing is mostly just not talking.
Si and Fe are easy for me to see working together because my mom is an ISFJ. My idyllizing looks just like hers, but it feels more "tragic" while hers seems to lift her up. I feel an almost permanent attachment to the idea that things used to be amazing and everyone in my life was friends and all was good, and if only it were like that again... Si also seems to back up Fe. "Nobody gets what's going on. Remember so-and-so? They always knew what to do. It's obviously everyone else's fault, not mine. Here are 30 examples why."