We could say, 'This experience [the one that is being described] should be happy and fulfilling. But very often, it isn't.' I hope that helps! Steve

Referring to something that a person might expect to be an enjoyable experience, for example starting university, a person might expect that this will be a positive experience, potentially fulfilling, satisfying, useful on a number of different levels etc. However someone might point out that many new students might experience anxiety, stress, financial problems etc, and see these as negatives. Hence a contrast between the expectation of a 'fulfilling and happy experience' and what may often - all too often means too frequently - and the reality of a possibly stressful and difficult experience.

皆さんのヘルプをまとめると、｢幸せで、充実した経験であるはずのものが、しばしばそうでないことが多い。｣ですかね。

I would first start by saying what is it that you are not understanding. English is very expressive and I find it uses a lot more vocabulary to express the same meaning other languages do in smaller structured sentences.