I found two phrasal verbs that are close to the meaning I want to convey:

1- Fit in: to feel that you belong to a particular group and are accepted by that group.

2- Blend into: to look or seem the same as surrounding people or things and therefore not be easily noticeable.

What I want to convey is blending into a community to the extent of losing one's characteristics and personality. It is as being too flexible that you can easily fit in in any community whether it was suitable for your nature or not.

I want to fill the verb in the blank of the following sentence:

Fitting in is one thing, but ______ communities is a totally different thing.

The phrasal verb "blend into" means "يختلط" in Arabic which also means "mix". The verb I'm looking for means "يذوب" which also means "dissolving/melting" but they don't seem (according to my research) to be used in English in that way.

"Assimilation" can work here:

assimilation (n): The absorption and integration of people, ideas, or culture into a wider society or culture.

Assimilation is a neutral term for a process that can be expressed either as a positive or a negative. To those in the wider culture, it may seem a good result to see some minority culture integrated into the majority -- but to those in the minority culture who would like to maintain their heritage, assimilation can be a worrisome.

As an example: For a long time the United States was thought of as a "melting pot" in which people of all nations, cultures, and creeds would mix together (like different metals when heated) to become one homogeneous whole. In this model, assimilation is viewed as a positive result.

More recently this was replaced with the model of "cultural pluralism" (also known as the "fruit salad" model) in which different cultures retain their uniqueness but still mix harmoniously with all the other unique cultural groups. In this model, assimilation is, perhaps, inevitable, but still viewed as a somewhat negative result.

(Edit) Because the noun assimilation exists, the gerund assimilating has a slightly different nuance that refers more to the process than the result. In your example sentence, I prefer to use "assimilation":

Fitting in is one thing, but assimilation is another.

However, when comparing two things, it's nice if you can phrase them similarly:

Accommodation is one thing, but assimilation is another.

or

Integration is one thing, but assimilation is another.

I don't have a simple verb to replace your blank, but consider going native:

Fitting in is one thing, but going native is a totally different thing.

From the Cambridge Dictionary:

go native

disapproving or humorous

​If a person who is in a foreign country goes native, they begin to live and/or dress like the people who live there.

Following up on a word mentioned by Andrew, you could consider homogenizing.

To homogenize something is to make it homogeneous: completely the same throughout, with no parts that are different from one another. It can be done to milk, for instance; all the little bits of fat get mixed in, producing a uniform liquid from which the fat doesn't separate.

Homogenize sort of has the connotation that the resulting product is bland and boring, so I think it fits your negative usage a little better.

I would suggest the phrase “losing your identity”. While it appears to be going out of style, the US at least has considered assimilation and blending in a good thing. It’s the great melting pot. So, those terms are going to be considered positive by some people.

Losing your identity, forgetting where you came from or forgetting your roots, are all seen as uniformly negative. Of these, I think the first is the most appropriate for your desired meaning.

To conform

Has there ever been a society which has died of dissent? Several have
died of conformity in our lifetime.

-- Jacob Bronowski

.

I think the reward for conformity is everyone likes you but yourself.

-- Rita Mae Brown

I think you actually want to shift the focus here.

Forgetting where you came from

The issue you want to emphasize is not the joining of the community, but the loss of the values and identity that someone learned in their original culture. In English, we're more explicit about this, since Western culture doesn't generally view adapting to the culture around you when you join it as a negative. Words like "assimilate" and "homogenize" won't quite convey the concept you want because they don't emphasize the loss to which you're trying to bring attention. Because they don't emphasize this point, they're going to sound awkward in your example context.

To "forget where you came from" is to abandon the values, beliefs, and distinctiveness of the culture you were raised in. It is a shift from participating in the culture you now live in to letting it determine your identity. It's a casual phrase (rather than formal), but it's also very negative. This seems to be what you're looking for. It can also imply that the listener may no longer care very much about the people they left behind, so be aware of that possible interpretation.

Fitting in is one thing, but forgetting where you came from is a totally different thing.

Note that it's technically ungrammatical; it ends in a preposition. However, in casual discussion, this will not be a problem, if the listener even notices it.