The issue is that as the jungler technically everything is your responsibility. You're supposed to help your lanes get ahead, you're supposed to help if they get behind, you're supposed to prevent the enemy jungler from having too much impact, you're supposed to get individually strong, you're supposed to shot call if nobody else is.
The reason jungling is considered the hardest role in the game is to be a very effective jungler you need to understand your champion, all the lanes, the champions in those lanes, what your champions can do, what theirs can do, when the lane can be successfully ganked, your risks if it gets counter ganked, what type of gank you can use, how long you should actually wait to attempt a gank, if it's worth expending your flash, when you're able to establish vision in certain areas, when you're able to go for certain objectives, etc.
The reason you feel underleveled in those situations is if a lane is losing, my assumption is the other laner picked up a kill, junglers are typically one level behind solo laners, if a laner picks up a kill on the other laner it typically causes a full level of difference between those two champions, between xp on kill as well as the extra xp from minions they're going to pick up that the other person can't.
That jungle guide VictoryDance96 linked is excellent, I highly recommend the read it's worth it, some of the things will be outdated of course, but a lot of the info is good. When you're learning the jungle it's better to play easier champions, sure flashy champions like Lee Sin are fun, but when you're trying to learn complex mechanical champions, as well as a supremely complex role, it makes it so much more difficult. There's no shame in starting out with champions like Xin Zhao, Warwick, Trundle, simple champions that don't have overly complicated mechanics to their kit, that allow you to jungle with ease, while learning how to properly influence the map. Frankly tanks are good to work with to start. Zac is a fantastic jungler, has amazing ganking potential, as well as a pretty fast clear.
With jungling and really with the game in general there's no one size fits all ever. There's a lot of decisions you're going to have to make constantly. Do I get a leash from top or bot? Do I need Blue to clear, or can I get away with starting on the side with Red Buff? Do I level 2 gank? Do I clear one side and gank? Do I do a full clear and gank? Do I full clear then back? Do I invade (Don't do this until you have some more experience it's super risky and can lose the game at level 1 if it backfires.)? Do I tower dive a champion (Also super risky and a little complicated to pull off)? When can I safely counter jungle? Should I invade for vision? Should I gank my winning lane? Should I abandon my losing lane (This is sometimes a fully valid plan.)? What do we need to do to press our advantage? What do we need to do to come back from where we're at? Really good junglers are capable of adapting to changing situations, but still some other junglers have a singular strategy, say camp a singular lane.
You'll have junglers who can gank anytime, and some who need to wait for a certain level to get a good advantage. Warwick has pretty pitiful ganks before he hits 6. Zac can gank as soon as he has Elastic Slingshot, which is often level 3.
Basically the best thing I can say is read a lot about the topic and the champions, watch videos, watch streams if possible, don't stick to a single person's stream, diversifying your knowledge from several junglers is very advantageous. Nightblue3 is a good one to watch and Valkrin is another excellent choice (though he's not a jungle only player.) They're both in the upper chunks of the playerbase, Valkrin is rank 19 challenger I believe, and Nightblue3 is a multiple season challenger and Master Tier player who plays almost exclusively jungle. You can watch a lot of other junglers too like C9 Rush, but Nightblue3 and Valkrin provide you with a lot of additional insight as they're both streams dedicated primarily to teaching the game, Valkrin can provide you with a lot of insight into other roles so you can start figuring out stuff about other lanes as well.
You also really need to have a thick skin, when things go wrong most players will immediately blame the jungler for anything that goes wrong, it happens, just keep your cool and mute them if necessary. It's supremely important to stay calm as the jungler and not let things bother you. You could get invaded and die early, just keep going, it sucks, but you can't let it get to you or you've already lost. I've personally used that strategy to my advantage, I had an early game jungler against Master Yi, he's weak early, I'm strong, I invaded him three different times, took both his buffs and killed him twice, he lost his head and his whole team tilted arguing with one another, it was a completely free game for me. It's a dick way to go about things, but it's also a strategy you can take sometimes.
Anyways, sorry for that massive post, but a lot of it should be helpful, some of it might create more questions, but that's good.
or 
fed and roaming quickly is going to snowball the game much faster than getting your 
or 
fed quickly. But sometimes you just have like a tank top and you wanna get your 
up and running ASAP, like i said it all varies game to game. To sum it up as basically as i can, identify your teams stong points as early as you can and support them. If it's 7 minutes in and your bot lane has died 5 times, bot lane no longer exists. Trying to gank that 5/0/0 
lane is probably gonna end up with you dead. But if you can get your mid lane 
just as fed, she's gonna take care of the fed 
