How can I learn a song fast .?

We need to look at your two examples for a moment before I can answer the question. Both are songs meant to be sung. Both are diatonic... what our instrument was meant to play. Your second example is a very traditional tune meant to be sung by children, with a structure of step-wise motion for the melody (conjunct) and a very typical rhythmic meter (rhyme scheme). This song, by its structural nature is easy to learn, play and remember.

The vocals for your first example does have a symmetrical rhyme scheme, but is the TAB for the lyrics?... if so, it doesn't seem to be very melodically accurate. Either way, the way the vocals are TAB'd, the notes are disjunct (has jumps), especially in the first line you provided. In this case, this song is harder to learn, play and remember. If the TAB'd line were simplified, this could be much easier. Please excuse me if I'm missing what was TAB'd there.

In regards to learning... that one is simple... Knowledge Begets Knowledge. Each song you learn develops specific skill sets that can be used for future learning. In other words, every song you study creates a subconscious knowledge base of movement, breathing, articulation, tongue location (e.i., tongue blocking, bending and tone), rhythm, etc. In other-other words, keep at it, this process gets faster over time. After a couple of years you'll start to surprise yourself how fast you'll be able to pickup some melodies (similar in some way to something you've learned in the past) and how long it will take to learn others (new material for your brain to build neural pathways for). Either way, it's a ride... enjoy the journey... it's a process we all go through... a process... not something to get over with as fast as possible.