It's okay to get lost sometimes but not to this extent.
This can be evident in the way you behave. Maybe you were a focused person but when you got into a relationship, you suddenly lost all the focus and rarely care about the things that were most important to you.
Yes, everyone changes but if it negatively affects your life then it may be time to re-assess your priorities and future in that relationship.
1.You give up your interests for theirs
Maybe you loved socialising and travelling but suddenly stopped after getting into a relationship because your partner doesn't like you doing that.
They have some form of possession over you and you let them do so because you love them and eventually give up whatever interests you have for them.
2.You regularly change your opinion for them to be happy
Even when you know that you are right, you still keep your opinions to yourself just to make them happy. You know that they won't nudge or try to listen to you when you try to give your side of the story.
3.You pass on opportunities for them
Maybe you have a great opportunity to go for a 3 month fellowship abroad. You give it up because your partner guilt trips you into not going or you don't want to leave them for that long. You are getting too lost into your relationship.