When to use magic? When there is honest need, secondary to need, honest desire (and if wise after a degree of honest measured consideration to intention, reasons and consequences), used when no other avenues are apparent or have seemingly been exhausted.
If you convince yourself you are not 'worthy' or it's impossible (even subconsciously) it won't happen - you are not receptive - same goes for a spell for someone who is resistant (it's unasked) or the feel they don't deserve it... Unless you start getting 'crafty'. Resistance can can cause backfires.
To avoid 'what you asked for but not what you want' use clauses in castings. Every positive has its negative.
Magic is a 'spiritual initiator' , a spell is 'cast' into the metaphysical lake waiting for the ripples of return. Once the spell is cast consider the outcome already determined - do not be impatient, or agonise, or repeat.
We don't live in a vaccume, while casting a spell is focused, once done, it is best accepted as waiting to happen and that all other 'dimensions' need to engaged in (as it ripples back to all those other dimensions) - focus on the desire and need - in manifesting a favourable outcome, not on the spell itself; like a net into the sea, it has already been 'cast'.
A job spell (for example) is best followed with a positive 'can do' attitude, concerted and renewed effort, taking advantages of turns of 'fortune' as she presents herself, carried by the quiet patient certainly that if possible the universe will conspire to go your way. So be careful what you wish for, who you wish for but accept some things cannot be effected or the return is to only be later recognised on reflection of its subtleties.
A spell for money? A little may come your way. A spell for the lottery jackpot? Hmm... Just how much competition do you think you have; that lies in many peoples wishes and desires.
'practice' is always a good reason, focusing on little things, incidental things for just such a purpose.
Magic, done well is an anonymous 'occulted' benefactor, a magical serendipity, a questionable happenstance. You will know, no one else need to, or vanity, pride and ego can cloud your basic connections.