Why do i always think and miss my ex boyfriend even when I have another person in my life now?

It sounds like you still have some sort of emotional connection to your ex boyfriend.  The thoughts back could definitely be effected by the similarities with your current boyfriend.  In many ways, it seems that you haven't really moved on.  To be healthy for most people, there needs to be a cut off after a romantic relationship that involves no contact, removing pictures, and removal on social media.  That allows a "clean break" in the relationship without the constant reminders of the past.  Your unwillingness or inability to do this raises some questions.  The desire to continue following them or have pictures with you and him together seems to suggest that you have had difficulty moving on. 

Some part of you may still want to be in this relationship as some part of you are still holding on.  If you have some attachment styles, it would be common to have this sort of response.  Anxious attachment is an insecure attachment with very strong emotional bonds.  When you have this type of attachment, you have a lot of emotional needs in a relationship.  You need lots of validation, reassurance, and usually affection.   You usually have a hard time trusting others, but very much want to stay connected and feel loved.  There is usually a lot of fear associated with the person leaving or abandoning you and any perceived rejction is really tough to deal with.  People with this type of attachment have a very difficult time with breakups and an even more difficult time with clean breaks of a relationship.  They often hold onto the idea of getting back with the person and have strong emotional feelings regarding the other person.   In many instances, the person may attempt to remain friends with the ex, even though this is impossible and unhealthy.   A person with anxious attachment will often quickly try to find another person to fill the "love hole" that remains after a breakup.  They often will attach with the new person and move quickly due to the emotional pain left from the first breakup.  The result can be getting involved with another while feelings remain to the first person.   Unable to resolve these feelings, the person often feels very confused about their current feelings. 

Avoidant attachment is another insecure type of attachment resulting from childhood experiences.  Almost opposite, the avoidant attachment person will move on more easily, but often will critique a new relationship and long for the past relationship.   It seems to me based on this little bit of information, that it is pretty clear that you have not moved on from your ex.   This new boyfriend may be an attachment or emotional bandaid when your true feelings have not been resolved with the other person.  At any rate, you need to explore what feelings may remain so you are completely fair to the person you are with.  It may be that you just need more closure and that you are truly committed to what you are currently in.  However, it may be that you have not moved on and are not ready to be committed to the current relationship.  If you do feel you are committed, you should remove the social media links and pictures so you can truly move on.  It is hard, if not impossible, to truly embrace the new when you are still holding onto the past.