It is interesting that in this western society, in which individual success is so important, that so many flinch at the thought of selfishness. When I say to be a little selfish I am not suggesting that you give up caring for others. I am suggesting a balance between giving to others and giving to yourself.
Here's my suggestion:
First of all give yourself time. This time can be something that you already have in your schedule or you may have to find the time. Now re-define this time to make some 'me' time. Start doing the things that you always tell yourself your going to do when you get around to it.
Start thinking about how you would treat yourself if you had yourself as a friend. If it helps, close your eyes and imagine an exact replica of yourself. Meet yourself. What do you think that she/he needs to be happy? If this were your friend what would you do with this person? Notice the qualities of this person that are likeable. If you were your own friend, what would you like the most about the friendship?
Make a weekly date with yourself and do something that you really want to do. Keep a journal and inside write something that you like about yourself every day.
If you are seeing yourself in a negative way, notice that but don't buy into that right now. Acknowledge that part of yourself, but remain focused on seeing yourself as the valuable person you are. Notice this and give yourself credit for not giving in to your inner critic. Know that that critical part of yourself has been put there to protect you from rejection, it has helped you to be prepared for others critical side. Tell that part of yourself that you see it's value but don't let that part of you take over your thinking.
Write every week about yourself, tell yourself what you did today that you did well. Make a weekly date that you do what you want to do. This weekly date is not about getting something finished, it is not about work. This weekly date is about spending time with yourself. Maybe go for a walk, go to a movie, do yoga, or anything else that you have wanted to try.
If you do this enough, eventually being alone will be a little gift to yourself. The quality of your relationships may improve because you don't depend on them as much to reflect back to you the positive parts of yourself. Instead you will already see the positive.