Is this OK:
I haven’t had feelings this strongly for someone in a long time.
And
I haven’t had feelings this strong for someone in a long time.
Use #2.
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In #2, it's the feeling that is strong. The adjective 'strong' modifies the noun 'feelings'. It's an easy and simple meaning.
In #1, the adverb 'strongly' modifies the verb. It means that it is the having that is strong. This is an awkward thing to say.
Clive