How would you differentiate a mature enterprise product that is similar to competitive products?

Great question. This is a traditional "positioning" problem most enterprises face with historically successful products. Before answering the question, for any behavior to change in such enterprises, all the cross functional stakeholders (product, sales, marketing, and engineering) need to recognize that the market has evolved (new competitors, alternatives, needs, or even customers) and the said product is not well positioned to be the leader forever. With that understanding, it is important to take a fresh look at the following:

1/ Who is the ideal customer "today"? If your product is still generating new revenue but the conversion rates are low, chances are there is a niche customer base now but our definitions are too broad for them. Historically your customers might have been early adopters, large enterprises seeking a comprehensive solution, or those valuing brand reputation. Today, they might have moved to 1/ Late majority who are risk-averse, 2/ niche sectors with specific needs, or 3/ decision makers who value different things. It is important to "redefine" them as specifically as possible
2/ What is the real "value" we provide? In this case, start asking the question without your product in mind. Focus on understanding, what is 1/ outcome your are providing, 2/ experience you are creating, or 3/ ecosystem you are neatly fitting into. There has to be at least 1 of them where "you are best in the world". It is really important to tie them to things your new ideal customer really cares about - security, effort, risk, scale etc.,
3/ What are the true alternatives? We tend to think every competitor (or only our competitors) as an alternative. Sometimes it can be an intern with google sheets (in case of data visualization software). High performing sales people truly understand the alternatives. It is important to dig them out to not mark every lost deal as being lost to a competitor. This happens because product and marketing teams' competitor lists are longer than an actual prospect's. Often, strong differentiators get ignored because we can identify a competitor that has the capability. But what if that competitor is not showing up on a prospect's list for other reasons.
4/ How are they paying for your product? Overall the purchasing behavior is also gravitating towards value based pricing than long term subscription models. Sometimes it is worth rethinking the "way" the product is priced than the actual pricing itself.
5/ How are they purchasing your product? It is also important to evaluate the channels of GTM. Maybe the buying behaviors shifted from direct sales to channel partners. Or maybe a preferred mode is to integrate your product into a broader solution offering with other vendors.

If none of these still work, the problem might be in product strategy itself and not positioning. Oftentimes I realized getting an external opinion (consultants like April Dunford) help make the process easier in a company than trying to change minds with legacy approach. Hope this helps.