My car won't go into 5th gear barely, I mostly have to force it in.

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I have a 1991 Honda Civic Dx sedan manual 5 speed. Only 100,700 miles and for about a month now it's been a pain to get in 5th gear. Sometimes it'll feel like it's in and then pops out and goes in neutral, sometimes it'll go right in to 5th no problem, and then mostly it just won't go into 5th at all and I have to just keep trying and trying either by taking my foot off the clutch when in neutral and just retry over and over until it goes, and sometimes I have to go back into 4th gear and try again. Sometimes it'll grind when it finally does go in 5th and it's worrying me. Also it started doing this right after I changed the gear shifter. I wouldn't see how that could cause it to do that, but it was almost immeadiatley after I changed it. Also I put an adapter piece on the stick part because the gear shifter didn't fit tight when first put on and that made it better for about a day or two but now it's just getting worse and worse everyday. I'm always scared I might mess my car up when having to go about 60 mph in 4th gear until it finally goes into 5th gear, having to keep trying over and over again, sometimes making me almost force it in and a couple times I had no other choice but to force it in. I'm really worried about this. It's never once caused me a problem until now and I've had it for almost 2 years, starting with 55,000 miles and tooken great care of. It basically was brand new when I got it. I know it's 25 years old but I feel like it would be a worse problem if it was because of age. It still runs great, and gets it's oil changed every 3,000 miles religiously. I've never had one problem with it for the two years I've had it. I love my car and hope to have it for about 3-5 more years if not longer because the low mileage and proper oil changes. Please help, I don't want to mess the gears up anymore than I've recently had to. I'm constantly worried and anxious about it, especially while driving. It's starting to stress me out. Could it be the clutch needing repair/replacement? I wouldn't think so because all the other gears are fine and my clutch seems normal, no changes felt in the pressure of the clutch or the height it goes up when taking my foot off. Everything else is or atleast seems normal and fair. Please help. My mom said it could be the synchronizer, does that sound right?