I am getting confused at the usage of the phrase "couldn't help myself."
For example, let's say I played soccer in the evening. What should I say?
I'm getting confused with two negatives in the same sentence.
Which one is correct usage?
The idiomatic form "I couldn't help myself [from] doing something" is usually used in contexts where the thing you did was of very short duration (you found you'd actually done it before you could think about the situation and refrain from doing it).
In practice what this means is that "I couldn't help myself from..." is normally followed by an "involuntary/instinctive" reaction - having some particular thought, laughing, sneezing, etc.
Playing football doesn't really fit such contexts for me, so it sounds slightly stilted. For more "premeditated" actions like that, I would say "I couldn't resist [doing something]".
Also, as pointed out elsewhere, since "I couldn't help myself" implies a "spontaneous" reaction, it's very unlikely you'd use it in the context of not doing something.
Neither is correct. I couldn't help myself is used as an excuse or a reason, so for your soccer example it would be:
"I played soccer in the evening because I couldn't help myself"
Even that is a bit odd, as I couldn't help myself is usually used to explain why we did do something, rather than why we didn't. A better phrase for this would be I couldn't stop myself.
"I couldn't stop myself from playing soccer."
or
"I played soccer in the evening because I couldn't stop myself"
The idiom can/could help Verb-ing is a Negative Polarity Item, which means it can only occur with a negative, but the negation is not part of the idiom.
The construction itself requires the verb phrase can help (or could help in the past), followed by a Gerund (-ing) clause as its object complement. Only those verbs can occur -- no other modals but can or could, and no other verb but help.
That's the idiomatic construction. It can optionally take a reflexive object
But normally this only occurs when the action that is being discussed is already part of the context, taken for granted, as in the answer to a question:
So the reflexive usually takes the place of the object gerund complement. If you want to include the complement, then you don't want the reflexive, normally. It's only there to add emphasis, and the idiom still means 'control oneself', whether the reflexive pronoun's there or not. And if it is there, it gets in the way; as you noticed.
So, instead of keeping the unnecessary reflexive and then horking the gerund clause in under some preposition -- which only adds syllables and complicates the parsing -- I'd just drop it all and say, simply,
instead of either (1) or (2) above.
Setting out the whole action with 'couldn't help...' seems rather clumsy to me. I think it works best with 'it':
'I'm sorry I lost my temper but I was so frazzled I couldn't help it.'
I couldn't help myself would be followed by an affirmative sentence.
However, as pointed out by other answers, your example does not really work because you usually can't help yourself doing something spontaneous, whereas playing football is a an activity.
