Learn Hardcore German: Meine Kollegin unterstützt mich, und ihre Gedanken helfen mir sehr. - My female colleague supports me, and her thoughts help me a lot.

Because the colleague is female. The suffix -in marks the feminine form: Kollege (male) → Kollegin (female). Also note the possessive: meine Kollegin (nominative feminine singular) because she is the subject of the sentence.

How would the sentence change if the colleague were male?

Mein Kollege unterstützt mich, und seine Gedanken helfen mir sehr.

Kollege instead of Kollegin.
sein- (his) replaces ihr- (her): seine Gedanken because Gedanken is plural.

Why is it mich in the first clause but mir in the second?

Different verbs require different cases:

unterstützen takes the accusative → mich (accusative of ich).
helfen takes the dative → mir (dative of ich). Other common dative verbs include: danken, folgen, gefallen, gehören, glauben, passen, schmecken, wehtun, zuhören.

Why is it helfen and not hilft?

Because the subject is plural: ihre Gedanken (her thoughts). The verb must agree with the subject:

Singular: Der Gedanke hilft mir.
Plural: Die Gedanken helfen mir.

Why is it ihre Gedanken and not ihr Gedanken or ihren Gedanken?

The subject is Gedanken (nominative plural), so the possessive takes the plural nominative ending: ihre.
ihr Gedanken is ungrammatical.
ihren Gedanken would be dative plural (e.g., mit ihren Gedanken) or accusative masculine singular in other contexts, neither of which fits here.

Does ihre mean her, their, or your? Which one is it here?

ihre can mean her or their depending on context.
Ihre with a capital I means your (formal). Here it means her, referring back to meine Kollegin. Also remember: the ending on ihr- agrees with the possessed noun (Gedanken, plural), not with the owner.

Do I need the comma before und?

It’s optional in modern German when und links two main clauses. Both are correct:

Meine Kollegin unterstützt mich, und ihre Gedanken helfen mir sehr.
Meine Kollegin unterstützt mich und ihre Gedanken helfen mir sehr. Writers often include the comma to highlight the clause boundary.

Does und change the word order like weil?

No. und is a coordinating conjunction and does not change word order. Each main clause keeps verb-second:

… unterstützt …
… Gedanken helfen … By contrast, weil sends the finite verb to the end of its clause.

Where should sehr go? Could I say sehr mir?

Put sehr near the end of the clause, after objects and before any final non-finite verb (there isn’t one here):

Ihre Gedanken helfen mir sehr. You can also front the dative for emphasis:
Mir helfen ihre Gedanken sehr. But sehr mir is not idiomatic in this sentence.

Can I rearrange the elements for emphasis?

Yes, German allows topicalization as long as the finite verb stays in second position:

Neutral: Ihre Gedanken helfen mir sehr.
Emphasis on the person helped: Mir helfen ihre Gedanken sehr.
Emphasis in clause 1: Mich unterstützt meine Kollegin.

Is die Gedanken von ihr acceptable instead of ihre Gedanken?

It’s understandable but less natural. Prefer:

ihre Gedanken (most natural), or
die Gedanken meiner Kollegin (genitive, also very natural).

Anything special about the noun Gedanke?

Yes. Gedanke is a so‑called weak masculine (an n‑noun). In the singular it adds -n/-en in all cases except nominative:

Nom. sg.: der Gedanke
Acc./Dat./Gen. sg.: den/dem/des Gedanken Plural: die Gedanken

How is unterstützt pronounced, and what’s with the umlaut?

ü is a fronted, rounded vowel (like the vowel in French “tu”); don’t pronounce it like English “oo.”
tz sounds like English “ts.”
Stress is on the last syllable: un-ter-stützt.

Can I say sehr viel instead of sehr?

Yes. … ihre Gedanken helfen mir sehr viel is fine and a bit stronger than sehr. You can also say … helfen mir wirklich sehr or … sind mir eine große Hilfe to vary the style.

Why is Meine capitalized but ihre is lowercase?

Meine is capitalized only because it starts the sentence. Otherwise possessives are lowercase. A capitalized Ihre mid-sentence would indicate formal “your.”