From Wiktionary, the free dictionary
deep inside (comparative more deep inside, superlative most deep inside)
- at heart, deep down
1998, Jon Land, Dead Simple, Forge, →ISBN, page 156:But deep inside he knew the truth was he longed for one last great adventure.
2008, Susan May Warren, Finding Stefanie, Tyndale, →ISBN, page 61:Besides, sometimes, deep inside, she longed to be twelve again.
2010, Judy Duarte, His, Hers and…Theirs?, Silhouette Books, →ISBN, page 45:He's been short-tempered and snappy for as long as I can remember. But for what it's worth—deep inside—he's a good man.