How do we know if it's ok to ask for help and that fears thoughts are valid enough when others suffer, how do we know that we are not just a burden?

It's okay to ask. The right person to ask help from for fears and troubling thoughts is a mental health provider. You will not be a burden - that's why they're there. Even if there are people worse off, we all hope for some less tragic or difficult cases. It may be different w/ friends/family who may not be able to help and may just feel scared and helpless. That, not you, would be the burden. Get the help you seek.