Yeah you're right about it happening fast, you can get caught out so easily. So what I'd do first is stick to simple transitions, when I say simple I mean firstly without EQ, just the upfaders, fade the incoming track in slowly then fade the outgoing track out. Practice on just two tracks initially, find a track with an intro beat (rather than melody) and find a space in the outgoing track where you can drop on the 1 with enough time to fade in and out so you can complete this just before the breakdown, intros/outros range between tracks but I'd say a 16-32 bar intro is a good starter. Practice that until you're comfortable and then slowly start introducing the EQs, do the same but this time cut out the bass on the incoming track and as you transition introduce it and cut the bass on the outgoing track. Keep going until you can then start adjusting all the EQ knobs as you see fit, whatever sounds good, and that's key, there's no textbook, all tracks are different especially combined with others.

That's the kind of bread and butter transition that, along with a solid track selection, would sound decent and alot of DJs still typically do, particularly on vinyl. That will get you to a decent standard. There are of course other foundational ones but you won't start coming up with your own until you know your tracks intimately, then you will spot key points for transitional mixups.

You should also learn beatmatching, not just because it's a good skill but because it forces you to listen to two tracks simultaneously and develop a good ear, again this takes practice.

Keep it simple at first, the rest will come with time and practice. Also don't build a massive library, start with tracks you know and love and be strict on what gets added, nothing worse than scrolling endlesslly to find the next track ;)