My advice

1. Don’t be afraid to go alone. You WILL have a good time on your own. DO NOT let ANY opportunity to travel slip by because of your friends lack of interest or commitment.
2. There isn’t a huge hurry. You might save $100 or so if you book your flight in the next month or so, but if you’re willing to risk that you can easily make all your plans a month in advance.
3. Plan YOUR trip. Send out the info to your friends and give them a “drop dead” date. I’d probably pick mid-April for your June option and a little later for your July option, Tell them you’re booking your flights on that date and DO IT. They can still join up later. You can still make some changes to the itinerary after that, but you’re at least on your way. Start booking hostels, buying rail pass, etc. a month before your departure date.
4. You’ll find friends as you travel if you stay in hostels. You can always sign up for city tours and day tours in each city as you go in the very unlikely event that you’re uncomfortable on your own.

P.S. You will regret it for years if you don’t take advantage of this opportunity to travel. You will probably have a few points on your solo trip where you will regret being alone — which may total a few hours. Compare years vs. hours of regret.

ditto. ditto .ditto