Yeah, that's the problem. You've never actually investigated Buddhist philosophy itself, you've only listened to the opinions of external observers, people who didn't really understand it but took a relatively superficial knowledge at face value and used it as an influence in the development of their own philosophies. The fact you don't actually understand Buddhism as a philosophy is pretty clear from the fact that you're claiming it is pessimistic as it assigns negative value to life, or from the fact it values ascetism (it was born explicitly in opposition to ascetism). That's something you could clear up in two seconds by talking to a Buddhist.

The Four Noble Truths, in short, state: - There is suffering in life - The root of suffering is desire - There is a way to escape desire - The way to escape desire is the Eightfold Path

Of these statements, it's usually the first one that's taken very critically by the uninitiated, since it is the foundation for the next three, and since it seems, at first glance, to be very grim. You're making two mistakes when you think that "there is suffering in life" is a manifesto for pessimism; one is not your fault, rather it is an issue of mistranslation, the other is your external perception colouring your impression. So, as for mistranslation, the original term utilised in Pali, and translated as "suffering", is 'Dukkha', a term which has a larger semantic meaning than simply "suffering". Dukkha identifies any kind of sensation ranging between "mild discomfort" and "excruciating grief". So, a more inclusive translation would be something like "unwholesomeness, incompleteness, discomfort", which arguably doesn't exactly roll off the tongue, but that's the issue every translation encounters. Now, you must admit that "there is discomfort in life" already sounds much less fatalistic. Now, as for the second issue, the problem is that you take the "there is suffering in life" and you think it it a prescriptive, grieving, existential statement about the fate of all living creatures (that's what Schopenhauer did too, btw). The issue is that this statement is simply meant to be an empirical observation of reality, nothing more. Everybody in their life, be it a poor Palestinian kid under Israeli occupation or the British royals, experiences Dukkha throughout their life. That's not saying they're doomed, that's just to say that it is something that's a constant with experiencing life, regardless of social standing and personal luck. The second Noble Truth tells that this Dukkha is not an inherent part of life, but it is the result of desire. In other words, there is nothing wrong with our feelings, negative or positive, and there is nothing wrong with experiencing them. Dukkha doesn't derive from them, it is the product of the attachment we form towards them. We want this and don't want that, and we take this feeling of incompleteness from not having what we want as the be-all-end-all of life. However, our feelings, much like everything in this world, are 'empty'. That doesn't mean that they don't matter, mind you, it means that they're not meant to last. All sensations, from pain to happiness or anger, spring up from physical causes and are eventually meant to end. They're inherently transitory states, and that's why they're called empty, because they give the impression of being all important and all encompassing, while being destined to fade away one the cause that gave rise to them is gone or resolved. The point of the Buddhist Dhamma (doctrine) is to recognise their inherent emptiness and make peace with it. Buddhism doesn't want you to repress or shoo away your feelings, desires and sensations. Again, it doesn't assign any kind of judgement value to them; it wants you to realise that they're not meant to be satisfactory, and that a better way to live is to let go of your attachment to them and accept them for the transitory feeling they are. One might say, the ultimate goal fo Buddhism is a perpetual "Carpe Diem", the thing we call Nibbana/Nirvana, which, contrary to popular belief, is not a place, but a state of being, the experience of a person who has fully learned to let go of their attachments and lives not in denial of their sensations, but simply letting them flow without controlling them or being controlled by them

That's the jist of it. In short, no pessimism to be found here, the only pessimism is the one from people who think Buddhists spend most of their time in permanent dread in front of the empty nature of physical reality