I think that definitely an ok thing to be feeling. Its like friends in real life, you can adore them and they can be right for you when you're in a certain stage of life, then as you grow and mature they become less and less what you "need." That doesn't mean you seriously don't like them or anything, it just means that you need to move on and grow apart, which is ok. Some people will always be perfect for you because that's just the way things are, but in most relationships an end will come. And it just sounds like the end of you and this particular horse has come. There's no shame in that. :)
My mare gets on my nerves regularly, I'm sure it's mutual. She sure knows how to drive me crazy, also probably mutual. However, she's my girl. She adores me and I adore her. Yeah, there are things about her that I would change, and if she were younger I might be inclined to sell her since I really would like something to work on. However, I made a commitment to her when I got her that I was going to keep her until she dies. Since she's 25 (23 when I got her) that seems like a reasonable promise. Especially since there really isn't a market for 25 year old trail ponies. :P In the mean time though, we're enjoying each other. I enjoy every ride with her because it's not every horse that you can take out on the trails, alone, for hours, all gaits, and not have to deal with one spook or unmanageable misbehavior. And I know that I got really lucky with her and whenever I end up riding another horse I always wish for her because she is so much more of a bomb horse than most. But at the same time, I often forget that I did get so lucky, and I feel like there might be something better out there, but then I come on here and read various threads and I am reminded of how she is so perfect for me.
I really do adore her. I started out with her when she was basically crazy, I really felt (and still do) that I was WAY in over my head. However, my trainer said that I could do it, and I guess she was right since we did do it. But yeah, I came to my Lacey girl with a severe fear of falling off horses, slipping horses, cantering, spooking, trail riding, excited horses, whinnying horses, and basically every other behavior that horses exhibit that could be construed as "slightly out of control." Well, Lacey did all of that. She'd slip, she'd spook, she was always excited, whinnied all the time, she even had a rearing problem. Well, needless to say, I got over my fears really quickly. The thing about Lacey though, is that I had all those fears and my two biggest were cantering and falling off. Well, she hasn't let me fall off yet (we've had quite a few close calls but she's always magically saved me) and she's never tried funny business at the canter (which was my fear). Once at a gallop, she bolted with me (she was so happy to be running, she was flagging her tail and just being delighted with running) and I was terrified. She had thrown her head up in the air so I had no control and I was sure we were going to die or something else terrible. But yknow what? For some reason I angrily yelled "LACEY!!!" at her and she came right back to me, she slowed down and got right back under control. That was a HUGE confidence builder for me, I mean, I had absolutely no physical control over her and she was loving running, yet she chose to slow down because I was scared.
She's taught me so much that would take all night to share so I won't but I'm a way better person for knowing her. I needed a confidence builder and she's become my confidence builder.
In any case, I think your feelings are very normal. :)
Fabio - 13 year old Arabian/Lipizzaner gelding
Hazel - 14 year old Angora goat
Atticus - 4 year old LaMancha/Alpine cross goat
Rest peacefully, Lacey.