Traditionally, forgetting sigils is considered absolutely necessary. You cannot bypass the conscious mind to work with the subconscious if your conscious mind is constantly popping in with reminders about what you want or why you want it or how much it sucks to not have it yet.
That said, forgetting is a task easier said than done for many people. For people who really struggle with this, my suggestion is to make multiple sigils a week and then pull them out at random to activate them after you’ve created a few without worrying about which sigil it is you’re activating.
The main thing is to just not think about what you’re trying to manifest while you’re activating the sigil or anytime you’re looking directly at the sigil. It can take some practice but it is absolutely possible. Giving yourself time in between the creation and activation of a sigil can help but with practice, most witches can train themselves to create a sigil and then immediately stop thinking about the intention so they can activate it.
Also, avoid waiting for the results of your sigil. This kind of waiting does nothing but remind you constantly of the purpose of your sigil which will slow down or even prevent your results.
If you really can’t forget the meaning of a sigil then try to approach it with positive indifference. In other words, don’t care about the results, the meaning, or anything else about the sigil that might pop into your mind. Cultivate as much indifference around the subject as possible and allow the thought to pass out of your mind as quickly as it came in.