Welcome to CA! First let me say that I'll pray for you :)
Now lets talk about unconditional love... Protestants are really big on talking about AGAPE love, the way God loves and how we should do this and I agree 100%.
However we have to remember that we are not God, we are only human and while we can strive for this we will often fall flat on our face.
As a father of 4 sons I'm doing my very best to not show favoritism, I truly love each and everyone of my sons the same, sometimes for different reasons but the same. However I'm quite sure they are going to grow and from time to time feel like I loved one of them more than the other.
Why? I would ask how do you know your mother loves your brother more? Did she say that? or is it because she gave or have given him things others didn't get? or let him get away with things others didn't do? Could it be she was trying to do better with the younger children? it could be... it could also be that she felt she was trying to make up for something or rewarding something he did.
I only have 1 sister but I can tell you since I graduated College it's pretty much been about her. There are many things that if I had done them my parents would have freaked, she got away with those with no consequence. This is still going on to this day, I did everything my parents ever asked and always went above and beyond while she often chose the other route and was appeased for doing so while I would have been out had I done that. Yet she feels that I'm the one who mom and dad loved the most because I may have gotten things she did not. What she continues to forget is that those things were dependent upon meeting some goal. I met those, I fulfilled what they asked and they kept their promises while she chose not to meet those goals... yet because she made poor decisions I'm supposedly the favorite. To this day I can do something or be ill and my mother will say "oh I'm sorry" but the least little thing that happens, when seems to be often, with my sister and it's the end of the world because no one had had to deal with that. When I explain that yea I had or did that, I just didn't complain the sentiment is just ignored because I don't understand... My sister and I have gotten to the point that we joke about this now, at least a little, because it's funny how we both see the other as being the favorite for other reasons.
Could this be part of the case? What does your older brother think? Does he think he was the favorite? or perhaps would he see some of the younger children as getting away with more.
I can tell you I'm often harder on my oldest because he is setting the example for his little brothers, at times that means he gets extra positive attention to. Not because I love him anymore but because if he is going to have the responsibility, he needs to understand that and be rewarded in some way for it.
Again, these are not love issues, these are dealing with the different curve balls life throws at parent while trying to raise their children. However I can definitely see where some children might perceive them as being about love. Remember though that only God's love is perfect, as humans we will judge from time to time, even when we try not to do so.
To say though that your mother not providing you unconditional love, when pretty much no one gets that, is somehow the issue with you not being able to commit to a relationship is psychological BS in my opinion. We have to realize that we are ultimately responsible for our own happiness, not our parents. Our life with others and with God were initially guided by our parents hands, for good or for ill, but as adults we have 2 choices, we can accept responsibility or try to blame others. Blaming others and not taking responsibility has become the norm in our culture, heh - look at the Bush administration, but that doesn't make it right nor will it ultimately lead to happiness.
What a wonderful gift it sounds like you gave your brother, it also sounds like you are a fantastic sibling whose mother I'm sure loves with all of her heart. God knows the things you have done and He knows how it hurts when others ignore those gifts and accomplishments. Deep down I'm sure your mothers knows and is very proud, she just probably isn't verbalizing it or showing it as she should.
We all need to feel that we have our parents approval on some level, no matter our age, if we are raised in a loving household because that approval actually matters. Sometimes though parents fail at this... for most I don't believe it's on purpose but rather their humanity and forgetfulness.
What you need to realize is how important and special you are, regardless of your mother's approval or unconditional love. I'm sure your bother isn't love unconditionally either, you just don't realize it. Know that you have given gifts and of yourself in ways few others can or ever will, God knows that and now I and others on this board know that.
God does love you unconditionally as only He can and He will never forget nor let you go. We might chose to turn from Him at times but He will never turn from us.
I would suggest you continue to pray about this and spend some time with a Catholic therapist to help you work through these issues. You sound like such a wonderful giving person, you deserve better than to feel this sadness which plagues your heart. As you can get through those feels, talk to your mom about it sometime, about how your felt and how you feel now.