Maybe. If you truly believe that they will, and have realistic goals, they will most likely work.
It's very possible that spells you write yourself could be more powerful than those written by someone else, no matter who they are, how powerful they may be, or how ancient the spell is.
When you write a spell, it has more meaning to you than something that was written by another. Therefore, you naturally put more energy and feeling into casting it, and it is essentially "better" than one you could have found in a book.
Some people just aren't as creative as others, and someone else's spells work just fine for them. There is nothing wrong with that. To each their own.