In the following I will call the direct object just ›object‹ and the indirect object ›adverbial phrase‹.
What you should know about verbs first is whether they're transitive, i.e. they can or must carry an object with them, or if they're intransitive.
When a verb is transitive, it can additionally have a dative clause, as you probably know:
Ichsubject
| schreibeverb
| dirdative
| einen Brief object
.
--› schreiben, trans. (+dat.)
Every time you find out a verb is transitive, the object part of the sentence is already filled (Ich schreibe einen Brief), so if you want to add something new you have to fill another clause: the adverbial phrase. It can be filled with a
– prepositional clause (Ich verschwende keinen Gedanken an dich), a
– dative clause (Ich schenke meinem Freund ein Andenken) or a
– genitive clause (Ich bezichtige den Angeklagten des Mordes).
This means that when you find an object and a reflexive pronoun in your sentence, the pronoun must be a dative:
Ichsubject
| macheverb
| mirdative
| viele Sorgenobject
.
Ersubject
| machtverb
| sichdative
| viele Sorgenobject
.
--› machen, trans. (+dat.reflx.)
If you don't see any objects, you can assume that the pronoun is an accusative object clause itself. Now we've got the object clause filled again (this time with a reflexive pronoun), so if we want to add something, we're gonna use an adverbial phrase:
Ichsubject
| fürchteverb
| michobject
| vor der Dunkelheitprep.clause
.
Ersubject
| fürchtetverb
| sichobject
| vor der Dunkelheitprep.clause
.
--› fürchten, trans.reflx. (+vor)
Summary:
– We know that ›machen‹ is a transitive verb and we can identify an non-pronoun object in the sentence (viele Sorgen): therefore the reflexive pronoun has to be a dative clause.
– We know that ›fürchten‹ is a transitive verb but can't find any non-pronoun object: therefore the reflexive pronoun is the object itself.
I hope I haven't missed the point completely and it does help you a bit.