shade

I mean if you do a front leg, sidekick (karate style) and it gets swept aside. Think "PKA kickboxing", you are very exposed. Since you also lack momentum you can't easily do a crocodile thrashes tail - as you can with a round kick. Samart actually did a seminar at Tiger a few years back where he focused on his side-teep. I'm not familiar with the fighting scene in canada, but I would guess it's the same as the fight scene elsewhere. Kids are mostly impacted by watching mma fights on youtube and they emulate what they see. Very few people actually take the time to watch Thais fight. There you see the side teep quite frequently.

Do you mean a sidekick or a teep with a slight angle (Samart style) ? Both are used quite frequently, but the sidekick leaves you pretty exposed if you miss.

Hitting the back of the head is illegal is most sports. This is also true of Muay Thai. However there is a workaround that is accepted and not penalised in any way. That is a simple head-kick, where the lower part of the shin hits the neck/head and your foot whips around to tap the back of the head. You can punch the neck as much as you want. Same goes for the ribs. There might be certain local limitations in some rulesets. So you just need to read the rules or go over them with the promoter/opponent/judge beforehand. Have fun

I don't really know how you teep, but if I might venture a guess, it sounds like either you're inflexible and can't teep high enough or, you might teep in a swinging fashion(think pendulum). Again, since I don't know how you're actually kicking this is only a guess. In any case, you can work in shadow on higher teeps, chest and face. Also work these kicks slowly(painfully slowly) so you'll strengthen your hips in the process. If you're unable to teep high enough on taller opponents, teep lower. Teep the thighs, if you do it right, it's really annoying. Also since most people step on every punch or kick, it pretty much stops them dead in their tracks and forces a reset every time. Have fun

Concussive blows, most definitely. Repeated touches (i.e. light and playful sparring) no, and there it is unlikely to found as well. Depression is actually quite common after a knock-out. If you've been around a gym with active fighters for a while I'm sure you've seen it. It might come off as lack of confidence after a loss, which might be a factor of course, but a more important variable is concussive blows. When the brain takes damage(concussions) your hormone production is affected as well as your perception. If you give your brain time to heal, it will usually pass in time. Unless you know what is happening though, it can be quite destructive to your relationships.