Well done for finishing it, @neumannsalva. I will answer your questions below ... they're just my opinion and I'm often a bit odd :D

First: should I shorten the sleeves?
Personally, I would. I think if it was me I would feel happier about it. I sense that you feel they're too long.
I normally just fold the cuffs as I'm so short I'm used to doing that ...

Second: which buttons should I use?
I like the ones with the stars actually or the metals ones :D

Third: why do I always find my handmade items ugly on myself?
I think that's because you may be too self-critical. It is good to be able to reflect and not just accept things, but you've done a good job with it so 'celebrate' :D

I feel it is a lovely cardigan and I like the lace motif on the front. It is also a good colour because you can combine it; perhaps you just need to wear it with a brighter colour top, eg mustard, lilac or a seasonal orange to make yourself feel happier about it.

** Did you use a smaller set of needles to make the neckline ? Perhaps it just needed a tighter tension that's all.

A row of crochet may be fine too but check the tension...