The fact that you have made FT8 contacts means that everything is "working". You don't say where you are but say that you can hear signals all over Europe, so I am going to assume that you are probably in Europe. You are fairly lucky in that regard, as there are many countries nearby that you will be able to hear, and once you have sorted out this issue should be able to contact too.
I would start with looking at the antenna. To be honest it sounds like your antenna is better than what I have (I live in a condo in a huge city), and I am also rarely able to make SSB contacts because I just don't put out a good signal. Yesterday I worked DU (Philippines) and YB (Indonesia) on SSB, although both took a little effort. The YB was working Europe (mostly DL), and initially thought I was further away than HS (which is where I am) because my signal is so weak.
If you can find an antenna you can experiment with, you might have more luck. At a QTH away from my condo, I have tried with literally a piece of wire attached to a 10m fibre-glass pole, with a balun and a counterpoise. That worked extremely well compared to the very compact antenna I have on my balcony. I made a few SSB contacts on 40m with that piece of wire.
If you can get yourself a balun, some reasonable wire and a 10m telescopic fibre-glass pole, you will have an absolute blast on 40m and 15m particularly. Just 10W will get you out remarkably far with a quarter-wave vertical on 40m - you will be amazed.
Power is not as relevant as people say it is. Most HF sets fall into two categories: QRP and 100W. If you are running 5W your signal is down 13dB compared to running 100W, and if you are running 10W it's down 10dB from 100W. One S-point is 6dB, so your signal is down only two S-points if you run 5W, compared to 100W. If you have 100W and someone says your signal is 57, then the same person would see you at 55 if you have 5W.
If you can't find a 10m fibre-glass pole (I bought mine from Sotabeams in the UK and they shipped it to Thailand just fine), then try using a catapult and firing your wire into a tall tree. A balun and counterpoises will help you with getting the SWR down and your signal up, and not having any common-mode currents on your coax - less of a problem at 5W but can be bad at 100W.
So I guess what I am saying is - antenna. It's always about the antenna. You could have a Xiegu HF radio, or an IC-7851. But plug it into a bad antenna, and you will be making the same number of contacts.
Also - keep going with the FT8! See which countries you can get on there, and what signal reports you are getting. If someone gives you a report of -21 and you give them a report of +10, then you can see the mismatch between your receive ability and your transmit ability right there. Your sent and received signal reports tell you everything you need to know.
Keep going! It can be very disheartening not to be able to make contacts, so please don't give up.