Can a pessimist try to be more of a "bright side" person without seeming fake?

Only way to do that is to try to be a little more optimistic thinking wise, but try to keep it real when you speak to others. If you’re usually the person who says “it won’t work”, “that would be boring”, or anything like that, try to adopt more of a “maybe” attitude to start off with. Say “it might work” or “how about we try and see?”. If you go on this agree-happy streak people will be thrown off by it, so unless you actually believe something is a great idea, don’t say it definitely is. xD

That all being said, if you’re going to change yourself, try to make this change for yourself first. I can understand that you don’t want to negatively impact anyone, but if this is who you are and you actually aren’t going to adopt any of these new views and thinking ways, it’s ultimately not going to benefit anyone for you to pretend that you are.

Being real and being yourself is the most important thing. It isn’t real unless you actually plan on accepting these changes within yourself, and no matter what, people will be able to tell that these efforts aren’t “you”. So, if you plan on being more positive, stick to it. They will adjust.