Learn Hardcore German: Es fällt mir schwer, alte Sorgen loszulassen. - It is hard for me to let go of old worries.

Here you no longer use es, because Alte Sorgen loszulassen is acting as the subject of fällt.
But in the original word order, you must keep Es.

Why is it mir and not mich?

Mir is the dative form of ich.
The verb pattern es fällt jemandem schwer (literally: “it falls to someone heavy”) always takes the dative for the person experiencing the difficulty.

mir (to me)
dir (to you, sing.)
ihm / ihr (to him / her)
uns
euch
ihnen / Ihnen

So:

Es fällt mir schwer … = “It is hard for me …”

Using mich (accusative) would be ungrammatical here.

Is fällt … schwer one verb like schwerfallen, or is it just fallen + schwer?

In usage, es fällt mir schwer functions as a fixed expression meaning “it is hard for me”. Grammatically:

fallen is the verb (here in 3rd person singular: fällt)
schwer is an adjective meaning “heavy / difficult”

There is also the lexical verb schwerfallen listed in dictionaries, but in normal writing it appears as:

Es fällt mir schwer, …
not
Es schwerfällt mir, …

So you can think of es fällt mir schwer as a set pattern you learn as a chunk:
es fällt jemandem schwer, etwas zu tun = “someone finds it hard to do something”.

Could I just say Es ist schwer, alte Sorgen loszulassen? What is the difference?

Yes, you can say:

Es ist schwer, alte Sorgen loszulassen.

This is correct and means essentially the same: “It is hard to let go of old worries.”

Differences in nuance:

Es ist schwer, … is more neutral and general: “It is hard (in general).”
Es fällt mir schwer, … emphasizes the subjective experience: I personally have difficulty doing this.

You can also combine both ideas:

Es ist schwer für mich, alte Sorgen loszulassen.
(very understandable, but stylistically a bit more “translated from English” than the very idiomatic Es fällt mir schwer…)

Why is there a comma before alte Sorgen loszulassen?

Alte Sorgen loszulassen is an infinitive clause with zu (“to let go of old worries”).

In modern German spelling:

A comma before zu + infinitive clauses is usually either required or at least strongly recommended, especially when the clause is clearly separate in meaning or has its own “mini-structure”.

So:

Es fällt mir schwer, alte Sorgen loszulassen. ✅

Here the comma cleanly separates the main clause from the infinitive clause:

Main clause: Es fällt mir schwer
Infinitive clause: alte Sorgen loszulassen

Why is loszulassen written as one word, and why is zu in the middle?

Loslassen is a separable verb:

infinitive: loslassen (“to let go”)
normal present tense:
- ich lasse los
- du lässt los
- er / sie / es lässt los
  etc.

With zu + infinitive, separable verbs put zu between the prefix and the verb stem:

los + zu + lassen → loszulassen
auf + zu + stehen → aufzustehen
an + zu + fangen → anzufangen

So we write:

Es fällt mir schwer, alte Sorgen loszulassen.
not
… zu loslassen ❌
… los zu lassen (archaic / nonstandard in modern spelling) ❌

Why is it alte Sorgen and not alten Sorgen?

This is about case, number, and adjective endings.

Sorgen here is accusative plural (direct object of loszulassen: “to let go of what? – old worries”).
There is no article (die, meine, etc.) before it.
With plural nouns without an article, adjectives take the strong ending -e in the accusative plural.

Patterns for plural, no article:

Nominative plural: alte Sorgen
Accusative plural: alte Sorgen

If there were a definite article, you would see -en:

die alten Sorgen (nom./acc. plural with die)

So:
alte Sorgen here is correct accusative plural with a strong adjective ending.

What exactly does Sorgen mean here? Is Sorge normally singular or plural?

die Sorge (singular) = “worry, concern”
die Sorgen (plural) = “worries, concerns, anxieties”

In this sentence, alte Sorgen = “old worries”, i.e. worries you’ve had for some time.

Some related expressions:

sich Sorgen machen (um …) = “to worry (about …)”
- Ich mache mir Sorgen um dich. – “I’m worried about you.”
ohne Sorge / sorgenfrei = “without worry / carefree”

Using plural Sorgen is very common when talking about emotional worries.

Could I say Es fällt mir schwer, meine alten Sorgen loszulassen? Would that change the meaning?

Yes, that is perfectly correct:

Es fällt mir schwer, meine alten Sorgen loszulassen.

The meaning changes slightly:

alte Sorgen = old worries (in general; could be “old worries” as a category)
meine alten Sorgen = my old worries (specifically the ones that belong to me)

Grammatically, meine works as a kind of article, so now the adjective takes the weak ending:

meine alten Sorgen (not meine alte Sorgen)

Is schwer the same as schwierig here?

In this context they are very close in meaning, but not always interchangeable.

schwer literally “heavy”; very often used figuratively as “hard, difficult”:
- Es fällt mir schwer, …
- Es ist schwer, das zu verstehen.
schwierig more like “complicated, problematic”:
- eine schwierige Aufgabe – a difficult/complicated task

You could say:

Es ist schwierig, alte Sorgen loszulassen.

This is fine and understandable.
However, the set phrase Es fällt mir schwer, … is fixed with schwer, not schwierig.
So you would not normally say Es fällt mir schwierig ❌.

Is this sentence reflexive in any way? I see mir and think of mich/mir as reflexive.

No, this is not a reflexive construction.

Reflexive would be something like Ich wasche mich. (“I wash myself.”)

Here, mir is just the dative object of the verb pattern es fällt jemandem schwer.

Es = dummy subject
fällt = verb
mir = dative person experiencing the difficulty
schwer = adjective
alte Sorgen loszulassen = infinitive clause (what is difficult)

So mir here is indirect object, not reflexive.

Can this sentence be used in other tenses, like the past? How would that look?

Yes, you can change the tense just like in English.

Present (now):
Es fällt mir schwer, alte Sorgen loszulassen.
→ “It is hard for me to let go of old worries (in general / right now).”
Perfect (completed in the past, spoken German):
Es ist mir immer schwer gefallen, alte Sorgen loszulassen.
→ “It has always been hard for me to let go of old worries.”
Präteritum (simple past, more written style):
Es fiel mir schwer, alte Sorgen loszulassen.
→ “It was hard for me to let go of old worries.”

The core pattern es fällt mir schwer, … just shifts its verb form: fällt / fiel / ist … gefallen, but the infinitive clause alte Sorgen loszulassen stays the same.