Oh love, he's done a number on you hasn't he? I'm afraid you have found yourself in an emotionally abusive relationship.
he has told me often I don’t dress ‘sexy’ etc and even shown me women online at the gym in tiny shorts and asked me to buy those shorts. Yesterday I wore an average pair of gym shorts to the gym, he told me I look ‘hoey’ and should respect myself as a mother and that he doesn’t want me to dress like that again…
He has no say on how you dress and shouldn't be commenting beyond "you look nice in that".
He's messing with your head by contradicting himself. I would even venture to say that he's enjoying it and seeing how much you can be controlled.
after returning from the gym he questions me on if any males have tried to talk to me or say hello (I don’t speak to anyone) he then asks if any male tried to smile at me too
Massive red flag. It's none of his business who you talk to and you can talk to whoever you like.
For the past 5 years now his phone has been hidden from me. I don’t know his password and if I were to walk in the room where his phone is he will run to grab it.
He obviously doesn't want you to see what he's up to on his phone.
we cannot follow each other on social media (particularly Instagram). He has had me blocked there for 5 years now.
My bet is that he's pretending to be single and chatting up other women.
he never posts me, even when we go on family days out etc he will post the kids and himself and I am always left out of it.
See above
He never texts me. The only time I receive a text from him is when he’s telling me to do something (eg cook something before he gets home or chore etc) - he messages other women via Instagram and WhatsApp everyday. His excuse is that it’s ‘business’ and they are just clients, but this has been happening for years. Yesterday I saw him on his dms and there were atleast 30 different female accounts he was talking to. This also bothers me as he is able to text other females but never me.
He doesn't text you because he already has you exactly where he wants you, at home cooking his dinner and looking after his kids. You are no longer of interest, sorry.
He doesn’t give me any attention. I understand he works hard but he never shows me any attention, doesn’t check in on me in the day, doesn’t talk to me unless it’s talking about himself/his day or something that he wants to talk about. He never makes me feel special. I don’t remember the last time he ever called me pretty or beautiful.
See above
He doesn’t make time for me. He had made a lot of effort for my birthday and Valentine’s Day however, on a day to day basis he doesn’t make time for me. I have asked for a his help in something in particular for just 1 hour where he works, and he has not done it for me yet. However he posts with other females (clients) and has managed to fit them into his calendar and help them…
See above
I am constantly lonely. I spend every day by myself, or with our children. Even when he is home I don’t receive the love or attention I want. I am always by myself and it really hurts my feelings.
I don't mean this harshly, but only you have the power to change this. He obviously has no wish to. May I suggest that one way to change this is to leave him and find someone who actually wants to me with you.
Now the last part… 2 weeks ago he told me I need to get an std test. This was very out of the blue and unexpected, Ofcourse I questioned why I would need to since we have been together so long? He refused to answer me but Ofcourse it’s obvious why. I did the test - negative. Now yesterday he keeps telling me to do the test again… when he says it he laughs like it is a joke. The first time he said he only told me to do it as a joke. Now this time I don’t know if he’s serious or not.
It wasn't a joke. I've never said this in almost 20 years of being on MN, but it is so so so obvious that he's cheating on you. He laughs because he can't believe that you fall for his lies. That he can tell you to take a test and you believe he's joking. If you believe he's joking, why do it? If he's not joking, why stay with him?
every day I feel so insecure, I feel as though I am always in competition with other women. Regardless of all of this, he ensures me often that I am the ‘best woman ever’ and how much he loves me and that he will marry me one day no doubt.
He won't marry you, I'd put money on it. He says what he has no to keep you at home looking after him while he lives a single man's life shagging around.
I feel like I’m being dragged along until he’s ready to settle down? Which should have been many years ago when we first got engaged/had children! I feel like I’ve become desensitised to some of these issues so it would be helpful to hear other peoples perspectives.
I'm really sorry OP, but I really don't think that he's going to settle down with you. Like you said, if he was going to do it, he'd have done it by now.
This is actually one of the clearest cases of emotional abuse I've ever read on here, I thought for a moment that you were winding us up, but then I remembered how abuse can build gradually, so you don't notice it until it becomes so bad you can't ignore it. I'm so sorry OP 
Please contact your local domestic abuse charity or Women's Aid.