I work as an application developer for a company that has one product: an administrative system for businesses. The company also uses its system. The company has a company-specific module that allows the administration of change requests. Anyone in the company can create a change request. Most change requests come from calls made to the helpdesk by customers, some change requests come from colleagues.
Being the most junior (in age and experience) developer and the only person working on change requests for the desktop application, I often get pressured to work on changes requested by (more senior) colleagues first, even though the external customers have requested changes much earlier. Our guidelines to picking up change requests are a tad vague: I should try to find a balance between impact, the time it takes and the age of the request, prioritizing easy, high impact, old requests. Colleagues usually request a convenience change for a company-specific module, which has little to show towards the external customers and arguably little impact in general.
I have an enormous backlog, so they may have to wait weeks for their change request to be finished, but so do customers. Their only potential argument is that we are also alpha testers of our own software and customers at least have to wait till the beta version releases.
How can I deal with these more senior colleagues (internal customers) asking me to prioritize them over external customers?