Can you walk me through the thought process of why a competitor existing means you abandon this idea? Are they so central to your industry that they can immediately lock up 100% of the market? Are they going to be cross-selling from something which is more widely distributed in your niche than say Quickbooks or Microsoft Office?
Don't compete on price. Enterprise customers care about it a lot less than you do, and enterprise customers are not motivated to purchase by "We saved a few thousand bucks and I lost my job because the deployment blew up in our face." I'd be far more worried about that sales objection than the existence of a competitor.
You can probably compete on many other axes. One of my competitors has 400 employees, at least 20 of whom answer phones with customer questions. I have 0 employees, intentionally don't have a routable phone number, and self-assess at mediocre in terms of responsiveness to email. And I win sales dogfights with that company, occasionally, because prospects believe I'll offer them better CS. (The winning argument, which I've stolen fragrantly from Jason Cohen, is "You can call them up at any hour, day or night, and instantly speak to someone who can't solve your problem. Or you can drop me an email, and it may take me two days to get to it, but your email will always be answered by the guy who built the system with his own hands. Your call who you want in charge of your questions when it is your business on the line.")
There are other options, such as competing on market segmentation. Your competitor, for example, might serve primarily a healthcare market. If the same need exists outside of healthcare, you can target your marketing/development over there, and make a product which really sings for those other audiences. Is their product generic? Make yours hyper-specific. Is their product hyper-customized? Make yours the "It only does 10% of what the other guy does, but it is the right 10%, and it actually works out of the box."
How do they market/sell it? Is it one of those "Ask for a quote and we'll let you speak to a sales rep?" type of deals. Consider selling via a lower-touch sales model. Worse comes to worse, you learn why high-touch sales is so darn popular.
Also, to impart on you as early as possible the Voice of Pained SaaS Founder Experience: pause building for a moment and verify that you can successfully sell this. If necessary, you can have mockups or a minimally functional prototype to support the sales conversation. There are many worthwhile SaaS products which cannot be sold by a single founder into particular enterprises, so knowing whether your product is saleable or not given your constraints is a useful sanity check on whether to spend the rest of the schedule building it out.
Selling SaaS which doesn't exist is fairly straightforward. Find a customer. Ask them to buy it. Note why they tell you "No." Adjust until you have gotten a "Yes." Now, repeat at least 5 times. Then, finish building it.
If you cannot find a customer, or you can find the customer but can't get the right decisionmaker internally to get the time of day, or your customer doesn't consider this a hair-on-fire priority, or your current conception of the product doesn't match things they budget for, or any of a thousand other things systematically happen to block sales, then building the software does not in itself cure those sales problems.
Incidentally, the number of enterprise deals which you have to close to have a very good living as a solo founder is somewhere between one and twenty. Their billion dollar valuation might be sustained by a team of stab-their-own-mother-for-a-commission-check reps being fed the Glengarry leads from the best marketing operation in enterprise software, but even they don't win every deal. Table scraps are delicious if the table is very big relative to the dog.