In the past, I had been concerned about "losing myself" in a relationship. Partners have expressed the same reservation: "I don't want to lose myself!". Years ago, I felt this as a real possibility, and I tried to be aware of any loss of self (as I understood it at the time). More recently, I considered the possibility less likely because I "have a strong sense of self"; how could I lose something so solid?
Now I've come to realize what I really meant by "losing" and what that self actually is. The self that changes in a relationship is the constructed, conditioned self -- basically, thought patterns, emotional responses, the physical body (to an extent). This incredibly malleable self changes all the time due to a vast network of conditions -- choices we make, effects from the environment -- although some aspects seem more solid than others. For instance, a person's morality or character can seem to remain constant over many years; it can even be apparent from a young age. Moral stances can and do change over time, although they might be more difficult to change by force of will. Other aspects of the self-in-flux change much quicker -- fashion, humor, language. But it's all up for alteration.
This conditioned self will change in any relationship. When you look into it, you can see the truth of the expression, "you are the company you keep". The conditioned "you" changes its conditioning with exposure to other conditioned selves. In large groups, the changes can be so drastic that someone may be driven to join a riot or a genocide, when they never would have considered it otherwise. When living with other people, the changes may be much more subtle, because they occur over a longer time. One may pick up destructive habits because one's friends or roommates are constantly bringing their own destructive habits to the relationship. The more intimate the relationship, the more possibility there is for significant changes to one's conditioning.
A "strong sense of self", in this situation, really means having solid boundaries. Someone with a strong sense of (conditioned self) could live among heroin addicts without becoming a heroin addict, while somebody with a weaker self would take on the destructive energy of the addicts. But what's going on is that the "stronger" self is continually renewing his past conditioning with willful choices (some people might say "grace" instead of will). He decides, "this is how I am", and renews that sense in relationship to others by using memory and the will to resist changing his own conditioning. In the case of living among addicts, building boundaries by renewing one's conditioning seems like a healthy form of protection against developing a destructive habit. But always maintaining strong boundaries doesn't work in an intimate relationship.
A trivial example of "losing one's self" in relationship might be changing one's taste in music. "Before I met you, I loved only death metal, but now, because all you listen to is bluegrass, I've lost my love for metal!" Someone with a strong sense of self might resist such a threat to his preferences -- "I like my music and hate your music, so let's just agree not to share music collections". A weaker self, who notices that he's lost his love of metal, may react by fretting over "losing his self" in the relationship, but that's just because he desires to have a stronger self! On the other hand, with the awareness that the conditioned self always changes, I can ask: how important is the particular conditioning that I'm defending? Maybe, when it comes to music, I won't have any boundaries, making those tastes totally open to change. Perhaps, if it comes to physical health, I'll resist taking on habits that I perceive as unhealthy; I'll place stronger boundaries around that conditioning. Or maybe it's the other way around.
In any case, I want to participate in the process of this changing self, with the total acceptance of inevitable change. Participating could mean being open and vulnerable at times while renewing the conditioning at other times. Two people in a relationship bring their conditioning (all 13 billion years of it!) to the situation, and then those two paths start to intertwine deeper and deeper. Memories become shared, tastes change, and values can change. All of that usually happens subconsciously, with more or less resistance. Without awareness of the malleable self, a relationship can easily become mired in conflict or just a low-energy replay of the same patterns, day in and day out. With awareness, intimacy, and participation, two constructed selves begin to reconstruct themselves, for the better. They each will always carry their individual conditioning, but where they meet and intertwine, a beautiful harmony can result. I would like to aim for this intimate harmony.
Of course, none of what I'm talking about pertains to the "true self" -- unconditioned, unborn, infinite. In that sense, when I used to pride myself on having a strong sense of self, that really meant having careful awareness of the conditioned self, combined with strong boundaries. But all of that is irrelevant with regard to knowledge of the true self, which is unknowable... (and the funny thing is that the true self of individuals in a relationship is identical -- since all conflicts are about conditioning, there can be no conflict in the realm of the unconditioned self. On the other hand, because variety is the spice of life, and conditioning=variety, we can't ignore the sort of conditioned self we're creating, in favor of some idealized union of capital-S Selves!)