It certainly does sound like a frustrating situation. I think I have been in a similar situation before. What I have tried to do is use statistics and quantitive feedback to prove where the problems lie. Sometime sit’s difficult to get this across, especially when people don’t understand the fundamental importance of design. Maybe you could find some examples of successful design to show them just how important it is?
I’m also working in a startup, and it’s a little frustrating because I don’t really get much feedback on my work, and so it’s difficult to validate any of my work from colleagues. What I did was use analytics and statistics to prove where pain points still existed in the design, and where we should improve. Essentially, as a sole designer, you have to be even more self-motivated, and find ways yourself to validate your design decisions. I found this really difficult because, as a young designer, you’re still learning as you go, and I had to spend extra time to learn about all these things.
As a result, design decisions are essentially left up to me, and then signed-off on by the business owner (in a bigger company that would the product manager I assume). I think herein lies the problem. The design decisions are ultimately up to someone who doesn’t know about design, or how it works. I have to spend so much time explaining my design decisions. For example, my client would often want a big, shiny interface with lots of videos, images and functions, and I had to explain and prove to him that this was probably not the right choice. I did this by showing him other examples, building and testing prototypes and showing him the statistics.
It is so frustrating as a sole designer, I know. Also, it can be difficult to stay motivated. To be honest, I think I could spend much of my days doing literally nothing, and taking the pay cheque, but I don’t want to do that. I want to be motivated and work hard on a project that I can really get involved in. This has meant I have to work extra hard in order to set my own goals, motivate myself and validate my own work. It’s not easy, but it will show in your work, and hopefully be seen by your colleagues.