I'm a boy but feeling like girl sometimes. I want to wear girl dresses and wig rarely. But I don't want to be a girl permanently. Please help me to overcome this?

Just because a lot of people identify as either male or female doesn’t mean that everyone does. If you’re realizing that you identify outside of the gender binary that’s great and you don’t have to “overcome it”. What’s important is that you’re being true to yourself and how you feel. If sometimes you feel female, embrace that. If other times you feel male, embrace that too. Same for if you don’t feel as male or female at all. Clothes don’t have to have a gender. If you want to wear a dress, that doesn’t mean it has to be a “girls dress”. If you own it than it’s YOUR dress and that’s what matters.

It sounds like your gender identity is non-binary. Being non-binary means not identifying 100% with one gender. You might feel like you belong to more than one gender, or maybe to one gender at a time and then another in a different time. This would explain why you feel like a girl sometimes, but not always, so you wouldn't want to be one gender permanently, because your gender identification is fluid. It's ok to be non-binary, it's still not a very widely known reality, but there are many non-binary people who live their identity freely and happily, according to their own needs and finding their own balance. You can do that too!

Gender identity and expression are two differents things. You can identify as one gender, but the way you are expressing yourself is linked to the definition the society you live in associate with another gender than the one you identify as. There's also the whole concept of gender fluidity, people that see themselves as fluide, genderwise will go from feeling to certain gender, to none to multiple ones at different moment in their life. It could also just be your artistic side talking. Clothes shouldn't be gendered. You should wear what you want. There's multiple explanations, but they all have one thing in comment: they are all valid and ok. There's nothing wrong with you.

This isn't something that needs to be overcome or fixed. It is who you are, and the best thing that you can do for yourself is to understand and accept yourself for who you are. It can be scary and difficult, but reading about others' experiences can really help with it. You can go on various websites such as the LGBTQIA Wiki to research about various identities. Some more well known ones that I think might be worth you looking into are genderfluid, bigender, pangender, non binary, and genderflux. It is also worth remembering that you don't need labels, and you don't owe anybody a label or an explanation of who you are, it is enough to simply be.