I know that being such a sentimental person benefits nothing for my future but I just can't let go of the past, even though it has terrible impacts on my metal health.

Attachment Tto Tthe Past

Short prepositions like "to" and short articles like "the" are generally not capitalised in titles, though this is but a minor issue.

Things from the past always make me feel emotional: whether it's a friendship that I lost years ago or just an old note from a classmate who probably wouldn't even recognize me now, it can make me feel a fervent longingness to relive that very moment again.

Just "longing" is the standard way to say it.

My mind often dwells on the past experiences and memories, always fantasizing about the idea of what if I could rewind the time (back).

(1) "The" might seem a little unnatural here, since it seems to me that you're just referring past experiences and memories in general, rather than specific ones.
(2) "Experience" should be plural, since you presumably have more than one past experience.
(3) "Back" might be redundant, since the direction is already implied in "rewind". That said, I don't necessarily think it's grammatically wrong.

I know that being such a sentimental person benefitdoes nothing for my future but I just can't let go of the past, even though it has terrible impact on my mental health.

Alternatively: "being such a sentimental person does not benefit my future (at all)". There are many other ways you could word this sentence and convey the same meaning.

Perhaps, I was so blinded by fantasy that I overlooked the threads of attachment that quietly tied themselves around my soul, until it began swallow me whole.

(1) The comma may be unncessary here. I personally think the sentence flows much more smoothly without it.
(2) The phrase is "be so X that Y". Another example would be "I'm so hungry that I might starve."

Things from the past always make me feel emotional: whether it's a friendship that I lost years ago or just an old note from a classmate who probably wouldn't even recognize me now, it can make me feel a fervent longingness to relive that very moment again.

My mind often dwells on themy past experiences and memories, always fantasizing about the idea of what if I could rewind the time back.

I know that being such a sentimental person benefits nothingis not beneficial for my future but I just can't let go of the past, even though it has a terrible impact on my mental health.

Perhaps, I was blinded by the fantasy that I overlooked the threads of attachment that quietly tied themselves around my soul, until it began to swallow me whole.