This can definitely be a very difficult situation to be in, especially as someone who is perhaps at the beginning of their development career. However the good news is that there are a lot of developers who are just like you!
When I first started at my last company I was the only developer, barely classed as a graduate as I was 'self-taught', two years later and I'm working at a level above almost all of my peers at a new company. In honesty, it's all thanks to the time I spent having to work it all out by myself to tight deadlines, with difficult technologies and awkward frameworks.
The way that I tackled my own situation, was that I got stuck into answering and asking questions on stack overflow. I started doing challenge tasks in my free time to become more familiar with the frameworks I worked with. I even started putting more time into reflecting on the weaknesses i had with the theories behind certain elements of computer science as a whole. (Including: MVC, design patterns, multi-threading, concurrency, etc.)
From what you've said your employer obviously doesn't want to spend more money than what they are currently paying you. It's understandable for companies to not commit to extra resources by getting more developers in than they think they need as they have their own financial priorities as a business. Unfortunately this can be quite a common occurrence in the industry. However you should also see this as a positive. You have the opportunity to set yourself against a challenge that not all developers get the chance to tackle.
Spend your time learning all that you can about the frameworks and technology that your employer uses, consider the kind of standards you should be adhering to in your code. Start to push back respectfully on deadlines you know to be unrealistic or challenging. You should try your hardest to have an honest and open dialog with your superior to help you complete your work to the best of your ability as a solo developer.
But the most important thing to remember is, don't let it stress you out you are obviously competent otherwise you wouldn't still be there. Take every task or problem one step at a time and figure it out as quickly and efficiently as you can. As you start to learn from google, stack overflow etc. You will find that you will get quicker at identifying issues and resolving them. You've been given a path with a steep learning curve, embrace it and become the best developer you can be!