Your comment "that wearing glasses it's actually more damaging as they force the eye to keep a constant eyesight), but really, how normal is this?" Is totally incorrect and false. Not sure where you read that but I would avoid going there again.
Glasses do not damage the eye in visual adults (after about age 7-8). Even is a prescription is wrong or incorrect it can cause discomfort and eye strain but no physical damage.
Changes in visual acuity during the eye is a common situation. Sometimes it occurs because the cornea is changing shape during sleep due to lack of oxygen, visual changes before and after wearing contact lens are common problems, vision also changes due to systemic problems like the fluctuating vision of a poorly controlled diabetic.
Your last paragraph sounds like presbyopia which is the loss of focus problem due to age. This would be far and away the most common problem if you are aged 38-45. It will only get worse with time till you need reading glasses or progressive bifocals.
See an ophthalmologist or optometrist for an eye exam.
Dry eyes are another common cause of minor variations in visual acuity, and in minor cases you may not notice a feeling they are dry. Lubricating eye drops may help. (though depending on the drop they may making things blurry for a moment before they spread out).
Your description of having problems focusing at distance after using near vision for a while sounds like it may be a minor issue they call pseduomyopia, or spasm of accommodation (those keywords should let you find more info easily) which can happen even in younger people. Essentially your eye muscles that change focus in your eye to allow it to focus up close get temporarily stuck and don't allow the eye immediately fully relax to see as well as they should at distance. It isn't a permanent change in your vision.
Wearing proper glasses doesn't damage adult eyes permanently. I don't know if you may be referring to the various debates they have over what causes development of nearsightedness in children, and whether changes in correction may influence that, but that relates to the growth of eye's in children, not fully grown adults. The issue of the cause of myopia is still not fully resolved as far as I know, but they seem to be leaning towards the major factor perhaps actually being the amount of sunlight a child gets.
oops, a typo, it is spelled pseudomyopia as in something that isn't really full myopia but only temporary.
I am under 30 years old.
SoftwareDeveloper, it's true that my loss of distance acuity is worse the more I look at close distance. My acuity also seem to be suffering if there is weaker lighting. For example my vision is a lot sharper when there is sunshine outside vs on a cloudy day.
You might also be right regarding dry eyes. I do use eye drops every morning as that is when I feel my eyes extremely dry (when I wake up). During the day I don't feel dryness so I don't use any. When I do tear my eyes (eg. I yawn) my vision does get a lot more sharper for a few seconds, but I assume it's because of the refraction that the tear produce. So at that time my eyes are no longer dry, but still does not offer better acuity after the few seconds I mentioned.
I just can't pinpoint what causes my vision acuity change. Randomly it seems some days I can see extremely sharp with my left eye, but then some other days, it gets even worse than my right eye (a lot less often, but definitely not as sharp compared to the "good" days). I also notice how if I have very little sleep (I wake up tired), my eyesight over the day seem more sharp. No clue why.