My background is in permaculture but there's significant overlap between that and solarpunk. My point of view is that permaculture and/or solarpunk work at the individual level. They work even better at the household level, and even better at the community level, even better nationally, and best internationally.

You don't have to change the whole world to be successful. You're not responsible for the entire world, only your own actions. So be a part of the solution, lead by example and persuade others to do the same. But you're not expected to carry 8 billion humans on your shoulders, all the other animals, the trees, the weight of all of the oceans, etc. People only believe this because it gets repeated incessantly but take a step back and realize how obvious it is that you can't be expected to be personally responsible for basically all of existence. You're not omnipotent. Let go of weird expectations that anyway are probably promoted by fossil fuel types to overwhelm people into inaction.

Be responsible for your own actions, be part of the solution, and let go of the rest.

I mean... I don't want to sound a smartass but it's the whole point of organizing. If one person alone can sabotage your actions are we sure that they are the issue? Just plan strategically aware of your powers for some achievable targets and so you can keep people who don't want that same target or ruin the plan away. The naiveté is thinking we can avoid using their tools against them or trying to do politics without getting our hands dirty with some incoherence or ''impurity''

in this answer there also could be a space for ''machiavellan'' leftists that find more barriers instead of allies because they are not pure to others but it would be a bit of shouting against the wind lol

it's sad having to think about politics like a strategy game? yes

do we have an alternative to have the future we want? not really

As someone casually familiar with solarpunk I would say it's strength is in painting a picture of what a better world would look like, and popularizing that picture. Which isn't to say that there won't always be negative people or opponents, but solarpunk will work (if it works) by rejecting the framing of naysayers and refusing to be contained by a narrow worldview/overton window.

Solarpunk can be both cultural, as in sci-fi worldbuilding, and activist, as in our individual everyday actions (I agree with the excellent comment from @LilNaib about the individual power we have - activist work should not leave people frustrated of even burnt-out). The activist part is the foundation of the worldbuilding ('How would the future look if people worked towards what I work towards'), and the worldbuilding part is the foundation of the activist work ('How do I want a better future to be like').

It's very tough to try to convince others of solarpunk, or anything, by commenting or posting online. Convincing happens, if at all, really slowly, by example rather than preaching. It can take generations. Right now we are a very sick society, slowly becoming self-aware, and if we manage to point ourselves into a good direction, we might live in a better future. But we cannot be this future now, or expect others to be this future now, it will take time.

You deal with them by being more numerous and making it count.

The antidote is democracy. The real thing, not the representative shit that is the norm nowadays.