keeping a friendship

I really like to meet new people and make friends, but the most difficult part about it is to keep in touch with all of them!

I think what makes a solid friendship is spending time together. Being trustworthy, supportive... these kinds of things demand time because it is they are built ed little by little.

Meanwhile time flies and now I'm 20 years old. I have lost a lot of friends that I made, but I'm not worried about it because it is common for people to come and go and back in our lives!

But, in fact, when I realize nowadays I have so many tools to keep in touch with them, sometimes I feel a little regretful to for losing contact.

In spite of these kinds of problems, I could can understand that mantaining a friendship is as important as making new friends.

This kind...

These kinds... 

Also consider using 'type'.

This type of thing... etc.

Build has the past participle  'built' 

'builded' can be seen in some old texts and poems etc. It is not in everyday use. 

'builted' does not exist.

I changed your text to 'come and go' because this is a common expression in English and therefore sounds very normal.

keeping a friendship

I really like to meet new people and make friends, but the most difficult part about it is to keep in touch with all of them!

I think what makes a solid friendship is spending time together. Being trustworthy, supportive... this type of things demand time because they are built little by little.

Meanwhile time flies and now I'm 20 years old, I have lost a lot of friends that I made, but I'm not worried about it because it is common for people to come and go in our lives!

But, in fact, when I realize nowadays I have so many tools to keep in touch with them sometimes I feel a little regretful for losing contact.

In spite of these kinds of problems I can understand that mantaining a friendship is as important as making new friends.